Electrical on old stuff is not fun. Kind of an electrical knowitall - worked in the City on Double Decker buses - 24volt, rusting, corroding British stuff made of many different materials. Installed 12volt audio systems in 24volt buses, fun stuff there. Sorted out electrical shifting Voith auto transmissions too. Learned a lot - the main thing is that if you touch it, you buy it. Yes, I can fix a bunch of old stuff, but might cause other future failures down the line so if possible, I ran new wiring - faster/more reliable too.
I can fix my own old crap, wouldn't do someone else's - too much time that is worth way more to me.
So: If you are having funky electrical gremlins, it's a ground problem or something hot is hitting a ground, maybe time for a new harness. Check every single ground & connector - clean & use electrical grease - that's what takes all the time. If it straight up doesn't work, probably a fuse or broken (like a bulb or radio or wire or relay is open) - then find problem n fix old wire or fuse or electrical googah.
Favorite electrical tool besides a good DVM: Powerprobe
https://www.tocber.com/w-case-circuit-resis...iagnostic-power Had people tell me they dont need such great tools, next time I see them, they're using a Powerprobe.