Replacing Engine Compartment Wiring Harnesses On A 1966 Ford Mustang – San Diego, California

     What a busy Friday. I had two front end electrical replacements today. I replaced the electrical from the firewall forward on 1966 Ford Mustang. The other one was same, but on a 1967 Ford Mustang that I will finish up on Saturday. Both of these Classic Mustangs had the original wiring harnesses. They suffered from brittle wiring, and several repairs over the years. After 40 plus years of use, it’s a good idea to inspect your electrical. Electrical shorts and fires can cause some serious damage to your investment.

     Before I replace the electrical harnesses, I check what works and what doesn’t. I also look at how alarm system, aftermarket stereo, or any other non-original accessories are hooked up. If any of those are to remain, I want to make sure not to disturb them.  If I see that they are not hooked up correctly, I will make sure they look like stock connections. The 66 Mustang had an old Clifford alarm that has not worked in years, so I removed it, and made sure there were no leftover hot wires.

     Once I checked everything, I removed all the original electrical harnesses in the engine compartment. I replaced the headlight harness, engine gauge harness, alternator harness, firewall ground strap, and headlights. I like to use Dielectric grease to help protect the connections in the engine compartment from moisture. The headlight harness not only connects  to the headlights, but it also connects to the front turn signals, voltage regulator, starter solenoid, and connects to the alternator harness. The engine gauge harnesses connects the engine sensors to the instrument gauge, heater blower motor, and provide 12 volt battery power to the ignition coil. The entire car is grounded through all the sheet metal, but the engine is sitting on rubber insulated mounts, so it is not grounded without the engine ground strap to the firewall.  This is important because the instrument gauges get their signal from a controlled ground through the temperature and oil  sending units. All of these electrical harnesses under the hood are exposed to years of heat and moisture, and are usually the first to go bad like on this 66 Mustang, and on the 67 Mustang that I’m currently working on.

     Once in awhile, I come across some bizarre stuff like the headlight bulbs on the 66 Mustang. The passenger side had a crack on the front it , so both of them were replaced. When I removed the passenger one it was split almost perfectly in half, but it still worked on the car. You’ll see it in the last picture.

If you’re having electrical issues, please give Classic Resto Garage a call at 619-929-8506 or email

Happy Cruising!

posted by inlinesix in 65-66 Mustang and have No Comments