I usually find grounding is the most often reason for such problems - and it's easiest to find out.
Using a "jumper-cable". Connected directly to battery -. Long enough to the cabin. Use a sharp screwdriver at the end to make contact. Grab through paint to reach metal. And just try if it changes if you contact dash frame or columns frame or body.
This is not the repair but you should make sure if it's a grounding problem or not.
If not, you can do the same on the +-side. Connect jumper cable to battery + and contact fuses holder.
