grounds
Otto, you probably need to attack this with a two step approach, both for grounds:
-1- Make sure your guage cluster has a good chassis ground. Right now it grounds through the wiper motor, so adding an additional wire is not a bad idea, particularly if the guages do any moving when the wipers, fan, lights, or defroster are turned on.
-2- Kadett lights, particularly the tail lights, use aluminum sockets. They tend to corrode over time and the connection gets to be not so good. I don't remember when the transition point was, but yours might even have the terrible rotating base for the taillights, which always seemed to have issues. The easiest fix I have found is to pull the bulb and run a battery terminal brush through the socket a couple of times, then apply a little petroleum jelly to the base of the bulb when you put it back in.
Those two Kadett specific "fixes" usually clear up lighting problems. Let us know what you come up with.