You might have a (partial) internal plate short in the battery. Got access to another batt.? This happened on aircraft once in awhile and drove us nuts in the Navy (due to corrosive environment). The - side becomes positive, But still LESS positive than the + side allowing 'normal' + to - flow for other functions on the car, but also allowing trace current flow to a ground point in reverse. The lamps attachment points or sending units act as a ground. (IE: Less positive than the voltage on the Neg. wire.) Imagine (or draw on paper) a + wire from the batt. going to a sending unit (switch that is open normally) Now put 12V on the Neg. wire coming the other way thru the lights. The attachment point on the 'downstream' side of the sending unit acts as a ground now, and the lamps stay lit. Oddly enough, when there was an actual problem, and the sending unit closed sometimes the lamps would go OFF because there was equal voltage on both sides! (No current flow) A + wire shorted to a - wire or attachment point in the guage cluster might cause this. GOOD LUCK! These kinda' elect. problems are frustrating. Try a different battery (Just jump to your wires after disconnecting them from your battery) first tho' to eliminate that posibillity. Then it will be a wire by wire trace job I'm afraid.
MCTOY
MCTOY