While the goal in any restoration should be to save and re-use as much as possible (the word is restoration, not replace-eration), there are certain items any sensible restorer will always replace. The wiring harness is one of those items. Until the mid 1950’s, wiring was insulated with a cloth material that eventually breaks down, particularly when it’s exposed to the harsh conditions and heat of the engine compartment. Like asbestos, the cloth covered wiring is usually okay if it’s completely left alone in a protected area, e.g. under the dash, but the moment you start messing with it to replace a switch, clean a connection, or even to replace a fuse, the brittle cloth insulation can start to flake off and expose bare wire.
In a car, exposed wire is NOT a good thing because the entire chassis and body is grounded to one terminal of the battery, so the moment one of those bare wires hits anything around it, you’ve got a short to ground. And a short to ground can burn your car TO the ground.
The Mustang used a plastic type “modern” insulation from the start, so brittle wire and insulation aren’t as much of a concern. In fact, the wiring itself looks to be in pretty decent shape. That being said, it’s a safety concern since the car sat outside for about 40 years, and as long as I have to remove the whole harness to prep the car for paint, I might as well replace it. Mustangs use several different sub-harnesses that plug into one another, and while I probably could mix and match and re-use parts of the old harness, it’s not worth taking the chance or fiddling with cleaning the various tiny sockets of each connector in the hope that the harness works out. And I’m certainly not about to sit down and test each wire for resistance and continuity when concours-correct replacement harnesses are available for relatively little money (by comparison, a complete wiring harness for the Packard Alec and I did at the ADM was $3000). When it comes time for my Packard to get some wiring work, which I fear will be the moment the Disgustang is done, I’ll probably have to take a different approach. Because the Packard has a completely functional harness right now, I’ll only need to replace the wires that are showing signs of decay, but that means a lot more work. For that I’ll need to unwrap each section of the loom and remove the bad wires, make replacements, and likely need to completely re-wrap the entire underhood harness. But that’s a project for another day.
Removing a wiring harness is actually pretty easy once the car has been basically stripped. I did it in an evening. The most important thing is to take detailed pictures noting each and every attachment point, routing, color of wire, and labeling the important connections. This may seem like overkill to the uninitiated, and some guys will question the necessity of documentation when you have the wiring diagram, but I know from experience how much time it saves to document the way things were. And no matter how many pictures I take, I always wish I had more. You can’t take too many pictures, or videos for that matter.