The Klima relay and OVP certainly play a role in the problems you're describing. I don't know why those relays would be missing from a car that has seen proper maintenance. First thing I'd do is make sure there is a good fuse in the OVP relay, then clean the sockets and pins of both relays. It should be easy to take a wire brush to the pins. I don't know how to clean the sockets. Maybe spray Caig Deoxit into the sockets. Hopefully that does it.

The reason for the AC compressor not coming on could be the used Klima relay. Maybe it's one that doesn't work with Diesels or there's an tired solder joint or two.

Tracing a loose wire in general is not easy. A really wild guess is that the voltage regulator brushes are worn and driving over bumps compromises the contacts. But if that were the case, you're have more problems than you describe when the power goes down. You'd probably see more warning lights than you see now.

Sixto
95 S420
87 300SDL

... and I'll say it one more time, an 87 SDL will not leave you stranded because of an engine electronics failure.