Mark Houlahan
Tech Editor, Mustang Monthly
May 10, 2018

When you really start to think about it, the typical classic Mustang is over 50 years old. While most of them have had some attention paid to the suspension, brakes, and whatnot, plus an engine rebuild or two and even fresh paint and weather-stripping applied, most are still rolling down the road with the original 50-plus-year-old wiring in them. That’s a scary thought! Add to that so many cars that have had wiring chopped up and improperly spliced for lighting, stereos, aftermarket ignitions, and more, and many of our classic Mustangs are simply electrical fires waiting to happen.

We chalk this up to many things. For one, wiring is boring, plain and simple. It doesn’t make the car go faster, stop quicker, or look better like the aforementioned upgrades. It’s also a bit intimidating for many shade tree DIY types. Most are competent enough to install a disc brake conversion or new headers, but when it comes to wiring aftermarket lights (for example), most DIY Mustang owners do not understand amperage, voltage, and resistance, and what improper wire diameter, poor connections, and more can do, or more accurately, not do. We’ve seen so many add-on upgrades without circuit protection (a basic inline fuse people—please!) that we sometimes have a second thought about riding in someone’s car!

Honestly, after 50 years on the road that stock wiring is brittle, corroded, and usually cut up and improperly spliced. The best solution is to completely rewire your Mustang from front to rear. We know it sounds intimidating, but we’ve got your back. Using a complete wiring solution like the American Autowire Classic Update Series kit we’re showing here takes 95 percent of the guesswork out of the job. American Autowire has already factored in the correct wire diameter, circuit protection, wire length, and more. Basically replacing your Mustang’s wiring is as simple as mounting the new fuse box, routing the wiring to the proper locations, and terminating a few ends. With the Classic Update Series many of the larger connectors, like ignition switch, headlight switch, and more are already terminated. You simply need to terminate lighting connections and your gauge cluster. Check out the photos to see just how easy this is, and then order your Classic Update harness and get busy!

1. The American Autowire wiring system includes the main dash/front harness prewired to the fuse box, a rear body harness, and large full-color instruction sheets.

2. Also included are most of the electrical switches the Mustang uses to ensure you’re not dealing with a 50-year-old switch with corrosion issues. You’ll find a new ignition switch, a headlight switch, doorjamb illumination switches, new taillight connectors, a high beam floor switch, and new underdash interior light housings. Ground wires, terminals, and electrical connector housings are all included as well. You’ll have to reuse your windshield wiper switch though, or purchase a new one.

3. This particular ’65 Mustang is in the middle of a fresh paintjob, so the interior and glass were out of the car at the time of the wiring project (perfect time if you ask us!). As such, the battery was already out of the car, but be sure to disconnect/remove the battery before you begin removal of the original wiring. We began under the hood by disconnecting the bulkhead plugs at the firewall and then removing all wiring from the front lights, starter solenoid (shown here), and engine.

4. The rear body harness travels down the left cowl side and under the driver-side doorsill plate to the rear. Since this wiring was already hacked up we simply cut the wiring at the doorsill and pulled it out from the trunk area.

5. The last bit of original wiring to remove was the underdash harness. If you’ve never removed the dash switches there is a trick or two to it all. Starting with the headlight switch you have to push the button in, shown here on the switch body, to remove the headlight switch knob/shaft. Then you can unscrew the bezel nut to remove the switch.

6. For the ignition switch, place the key in “ACC” and then insert a small paperclip or pick tool into the release hole while turning the switch counterclockwise past “ACC” to remove the tumbler. Finally, push the ignition switch body toward the dash face and rotate to disconnect it from the dash bezel. The cigar lighter and wiper switch are simply unplugged and left in place.

7. After extracting the rest of the underdash harness, along with several owner-installed wires, auxiliary fuse connections, and old unused gauge wiring, etc. this is what we had on the shop floor from this ’65 Mustang. A real mishmash of original wiring, aftermarket wiring, fuses, relays, wire taps, and more that we’re not unhappy about seeing go away. Ironically, this car is owned by a firefighter, and we couldn’t help but comment about what a fire hazard his wiring was!

8. The American Autowire kit includes two grommets for the dash wiring pass-through points for the engine compartment and front lighting wiring. They simply press into place of the old bulkhead connectors. It is advised once all wiring is complete to apply some sealer around these and the wiring itself.

9. The kit includes this J-clamp that is to be mounted to the bottom of the cowl brace where the brake pedal support attaches. This clamp will support the main harness as it comes up out of the top of the fuse box and routes over said pedal support.

10. The included template can be used on the engine side or interior side of the firewall, whichever is easier. Since this ’65 has an inline-six and the hood was off it was easier for us to use the engine-side template.

11. Once thing we didn’t anticipate was the Lokar cable throttle conversion interfering with our fuse box mounting. We ended up drilling a new pair of holes about 3/8-inch higher to allow the cable and pedal to clear the fuse box without issue.

12. It takes a little effort to get the wires routed over the pedal support, but begin by cutting the large cable ties and unrolling the wire lengths to determine what wires go where. This first bundle is the main power lead to the fuse box and our starter wiring from the ignition switch to the starter solenoid.

13. The main dash wiring is routed over the pedal support and up to the open dash area for now. We’ll leave the large tie-wraps in place to keep the wire bundle neat and out of the way as much as possible until we’re ready to tackle that section.

14. Here’s the front wiring roughed in. This is all of the wiring for the engine, lights, starting, charging, and more. Some wires you may not need (we’re using a Dakota Digital dash, so we stripped out the water temp and oil pressure sending unit wiring), and now is a good time to “diet” those from the harness as needed.

15. The rear body harness must pass through the cowl side and down the doorsill area and through the rear quarter-panel access area on its way to the trunk. The wires are all different lengths due to their routing location, so it is best to use small pieces of electrical tape at each wire end to secure it to an adjoining wire. This will allow the harness to pull through the access openings more easily.

16. Speaking of access openings, when going from the doorsill area into the front of the rear quarter-panel there is a circular opening with a plastic trim ring that is used for wire routing. Be sure to use this area to prevent wire chafing on raw metal edges.

17. Now that we have the majority of the harness routed and roughed into place we can concentrate on making connections, cutting the harness to length, crimping connectors on, and wrapping the finished harness. Here we’re verifying the distributor wiring by inspecting the wire printing and then cutting it to the proper length.

18. As the harness gets cut to length and final routing determined we cut the small wire ties away from the harness after adding a loop of electrical tape adjacent to the tie-wrap. This will allow us to more easily cover the harness in split loom or harness wrap without the tie-wraps catching on everything or causing a “bump” in the harness.

19. With the harness routing confirmed and each wire routed and cut to length it’s time to install any loose connectors included in the kit, such as headlight, parking light, horn, etc. If you pay attention to the wiring diagram you will see several wires that are used for low beam, high beam, horn, etc. that route to one connection point and then continue to another. Those first connections where you will be crimping two wires together take the larger/wider terminal shown here to the right. These are for the headlight connection.

20. This is an example of a completed connection where the wire, in this case for the horn, goes into the connector and then comes back “out” to run to the other horn assembly. When making this kind of connection be sure to use the wider terminal that accepts two wires.

21. Sometimes you’ll come across a connection that the American Autowire harness does not have the applicable connector for. Such was the case with the backup light connector on the T-5 five-speed that had been previously swapped into the Mustang. We’ll probably order the proper connector in the future, but for now to get our backup lights wired we scavenged the two push terminals from the starter solenoid wiring harness to make it work.

22. Here’s our finished engine bay and headlight wiring all routed, covered, and secured with OE wire loom retainers. The cloth split loom gives a vintage look and in our opinion is more fitting than plastic split loom when a vintage engine is underhood. If we were wiring a car with a Coyote or some other late-model engine swap, we would most likely use harness tape–wrapped plastic split loom.

23. The rear body harness was taped and wrapped in the same manner after confirming wire routing and individual wire length. As up front, the rear parking light wire will need to be daisy chained from the left taillight to the license plate light and then on to the passenger taillight.

24. American Autowire includes this nice molded wire grommet for the fuel level sending unit wire. Just like flaring a brake line, be sure to route the wire through the grommet before adding the wire connector on the end of the wire run.

25. While the American Autowire kit includes taillight pigtails for a plug-and-play solution with stock taillight housings, this ’65 is running Mustangs to Fear sequential LED Shelby-style taillights. No worries though, we simply used the female wiring connector and crimped the appropriate terminals to the taillight’s wiring to make it a nice and clean direct plug-in with the ground wire connected separately, as shown here.

26. New door pin switches are included and the wiring harness is already terminated to the right length and with the proper terminals to simply plug into the switch and then seat the switch in the doorjamb. Note the harness includes extra wiring for a dome light circuit, which is great if you added a dome light or need the wiring for fastback interior lights, etc.

27. With the front and rear harnesses terminated, wrapped, and connected, we made the final push to the finish line with the dash wiring. The new ignition switch simply plugs in and the accessory wire is connected to the rear stud. The new switch installs just like the original one with the existing bezel and rear dash “cup” used.

28. The new headlight switch is the same process. Plug the pre-terminated headlight connector into the new switch, seat the switch into the dash cavity, and reinstall the headlight bezel and retainer.

29. The steering column wiring connector is pre-terminated on the harness side and American Autowire includes the mating connector and terminals to wire the OE column wiring along with a diagram that is easy to understand. If you have an aftermarket column that uses the long GM wiring connector it will simply plug right in.

30. Our last bit of wiring is the gauge cluster. As mentioned previously, this ’65 is using a Dakota Digital VHX gauge cluster that utilizes a separate control box that “talks” to the gauge cluster via a network cable. As such, we simply had to cut the gauge wires to length and secure them under the proper screw terminals of the Dakota Digital box. However, rest assured, the American Autowire kit includes a nice male/female connector and terminal kit to create a quick disconnect setup for the stock gauge cluster. Note though that American Autowire does not support the OE ammeter, so that’ll be bypassed and non-functional in a stock ’65-’66 five-dial cluster.

31. The last step is to reconnect/install the battery and power up each circuit and verify everything works as it should. We started with items that didn’t need key power first (headlights, horn, brake lights, etc.) and then turned the key on and checked gauges, radio, etc. before actually starting the car. Good news! All wired and not a single issue. Time to finish assembly of the freshly painted fenders/hood and get this ’65 back on the road!