I had a '70 Chevy pickup that had a weird problem that I fought for years. It only happened at night if it was raining.
I would start it up, turn on the headlights and start driving. When I turned on the windshield wipers, the whole electrical system would shut down, and the engine would stop running. Turn off the wipers and everything is back to normal. Or sometimes turn on the wipers, and it runs as normal. (that is what taught me that if I do something that has negative consequences, undo what I did)
Like I said,I fought it for years before I found out what was wrong. The accessory wire that came from the positive cable on the battery connection went to a connection on the fender. That connection had an eyelet on the end of the wire. That eyelet was broken under the plastic insulator. So sometimes it made a connection, and sometimes it wasn't connected enough to carry the current when everything was on.
I discovered it by accident when changing the battery. I accidentally pulled on that wire and it came from together. After looking at the corrosion on the two ends, it had been broken for quite a while. I replaced the eyelet and never had that problem again.
Several years later a friend had a restored '68 Chevy pickup, and he told me about a problem it had. Sometimes it would start, then other times it would not even turn over. No headlights, no radio, nothing. Could not even jump start it off another vehicle. I went over and replaced that same eyelet and fixed his problem. After that he thought I was the most brilliant person to ever work on old Chevy trucks...