back feeding - if a wiring fault occurs like losing an earth, the electrical current finds alternative paths to make a circuit, a bit like when you see the rear lights on a car all glowing dimly when they brake or whatever, the earth is lost so it finds an alternative route across the other circuits and all the bulbs do strange things as a result.

I couldnt possibly comment mate, needle in a haystack. Checking the connections on the instruments themselves would probably be a good start.