I had a 1980 year Oldsmobile 98 that didn’t have as many crazy issues as yours but did have one amazing one.

Driving home one evening from college classes the headlights didn’t work so I took it into the shop.

Couldn’t find anything normal as a cause but I had one of those old time small town mechanics that couldn’t stand to lose to the car. So he said he wouldn’t charge us for the extra work hours if he could keep it as a project until he was done. Took over three weeks of him going through the wiring and finally found a harness/wire that had worn through and was grounding to the car frame.

So far nothing too weird for an old car. The bizarre part is that he had good current equipment and it is supposed to test if a wire is grounded out like that to the frame or even if it is broken by kicking signals along it like you can to find damage to Ethernet cables.

So with that tester in hand and knowing without question what the problem was he hooked it back up and it still reported nothing wrong. He called the manufacturer and they said as far as they know that violates the laws of electricity… Worked fine with the new wires so again definitely correct and his tool worked on everything else he ever tried it on.