Chances are there are no wires on the back of the gauge. Connections are made by an overlay. How full does it read? My sending unit went bad one time and the gauge went so far to full side that I couldn't even see the needle. New sending unit and all was better. There is a ground wire that runs from the sending unit to the frame that sometimes gets coroded and loses contact. If you get under and look up, pass side front of the gas tank near the corner, you sould see it.
