We just had a similar issue with our 1994 Jetta GLS. We had weird stuff happening and through taking stuff apart and checking wires, we found out that the wires in the left rear door were severed, probably because of the poor design of when you open and close the rear door, the way the wires are set, the door severed them. We just ended up splicing the wires and using butt connectors to fix the problem ourselves rather than taking it to a shop and paying top dollar for them to fix it. I hope this helps. Also, check the wiring harness. You might find the problem there as well, and sadly people rarely check there.
