The best advice on wiring that I read on this forum was to take the fuse block out, unplug the wires from the back, take the glass fuses out of the blade holders and take a small round file and clean up the contacts where the fuses touch the blades. Also check the continuity of each fuse before you put it back in. Try to shine up the contacts where the plugs go into the back of the fuse block also.
Your contacts are 30 years old and could probably stand a polishing.
I also agree with everyone else that it is a ground issue but I would do the above mentioned maintenance before proceeding to anything else.
Oh, and by the way, Welcome
!
Copenhagen1