I would bet that it had been worked on by someone who neglected to make/remake a connection when they worked on it. What make/model- do you remember?

I was visiting friends in PHX- a friend who had replaced the engine in a truck owned by his future father-in-law's company had noticed some problems afterward. The lights would stop working when the clutch was used and when th wipers were used, the horn would honk. Since he hadn't done anything to the wiring for these accessories and none of the wiring was pinched, it was baffling. He has serviced vehicles for decades and has extensive training, I had done 12V electrical installations for ten years and the other guy was an electrical service tech, so it should have been easy enough to find, right? It was decided that we needed to lok for a broken wire, so I headed into the building for a spool of some kind of wire to use as a jumper- anything larger than 14Ga would have worked safely. As I returned, one of them was in the driver's seat while the other looked under the hood and checking for anything that could cause this weirdness and when I heard the horn, I looked up to see that as he engaged the clutch, the truck's cab would rise and when he took his foot off, the cab lowered. Being a heavy duty truck, the clutch had a strong spring, so it took a bit of effort and it was strong enough to exert a lot of force on anything associated with the linkage. Like the cab. Which had some underbody rust from its time spent in places where snow and salted roads were common.

Turns out, salt and time had caused the area around one of the body bolts to disintegrate and since the accessories are all grounded to the body and the battery had originally been grounded to the frame, lifting the cab affected the return path and made life a bit interesting. It also didn't have a connection directly to the body, so I made a jumper to connect this and the problems ended.