Hey guys,

thank you for your replies which I appreciate very much.
Yesterday I checked my trunk-wiring-loom. Though there were some cables with a broken insulation, the wires themselves were intact on all cables. The cracks in the insulation are so tiny that I don't believe that a short would be possible. I'll still insulate all of them properly, though.
For now I have reassembled my car's interior and kept the GM uninstalled. This way I can drive the car, though very limited.
Honestly, I could live without the power windows and the central locking. The thing that gives me headaches is the windshield wipers. There's no way to use the car without them since Germany is a very rainy country.

My dad proposed that we bypass the motor of the wipers and install a switch on the inside of the car. Does anyone have experiences with laying cables from the engine bay to the interior?

My next guess would be the doors. Maybe there's a short in one of the door-wiring-looms.

I also bought a used crash sensor (Stoßschalter) and will replace it as soona s it has arrived.

Until then, best wishes.