We all have our black boxes—things we’d rather not tinker with because we don’t know how they work and don’t want to know how they work. As long as the black box is doing what it’s supposed to do, it’s easy enough to leave it alone and focus our resources on other things. If it’s not working, we replace it or hand it off to somebody who knows what they’re doing. Having black boxes is nothing to be ashamed of; it’s just an acknowledgement of your limitations.

Electro-modding an old car—specifically, thinning out a wiring harness—will quickly reveal every black box in your life, both literally and figuratively.

Thinning out a donor car’s wiring harness is a matter of removing unnecessary circuits before transplanting the bundle into your project car. In my case, for my Chenowth EV, I won’t need power door locks, power windows, or a rear defroster, so why keep any extraneous wiring? It adds  weight and complexity, but more importantly, the less wiring I have to sort through, the easier it’ll be to diagnose the issues that will inevitably crop up when I get to the put-it-back-together-and-make-it-work stage.

But, as with any modern vehicle, eliminating wires is not easy. Black boxes abound. Circuits might meander from one end of the harness to the next. The wiring might be discrete, but the functions it carries out are often far from obvious, even if you have a factory wiring diagram.

Still, it needed to be done, especially if I wanted to continue with my plan of using all of the stock components necessary to drive the traction motor from the donor Nissan Leaf using the Leaf’s traction battery. So, to make sure I didn’t screw up anything or get overwhelmed, I developed a few strategies for this weeks-long process.

1. Label everything. I’ve seen people use subtler labels than green masking tape, but subtle would be self-defeating to me during this process. I needed to see at a glance what a group of wires did and where they terminated. Even when I identified and removed an unnecessary circuit, I labeled it where I clipped it (and on both sides of the connector, if it ran through one), just in case I needed to refer to that wire stub later on

.Just as important, I removed pretty much everything that the wiring harness plugged into from the car, even if I knew I wouldn’t end up using it. That air-conditioning compressor above obviously makes no sense in a sand rail but still serves as a good reference. This strategy has proven particularly useful for the steering column and the various controls mounted to it.

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