I have a classic from '82. The wiring harness is a mess, at some point people all kind of accessories to the standard wiring, including a second switch for the main head lights and a totally illegal boat horn with a separate switch to turn it off. I figured out the horn and the head lights, since then I decided to not touch it as long as it works. The car has all of three fuses, no way to have a simpler harness. I once asked a garage, for what turned out to be a faulty relay, and I got a conservative estimate of 2k just for diagnostics. It took me almost two weekends to figure the relay out by sheer chance.

Replacing, and diagnosing, a modern cars harness for 5k is actually quite reasonable if not even cheap. Especially since the harness rebuild means basically disassembling the majority of the car simply to get access.