Join Date: Aug 1999
Location: Ontario, Canada
It sounds like the circuits are some where inter connected, I would start by checking the wiring harness in the console, some body may have grounded the gauges with the courtesy lamp at the back of the console.
To check the gauge operation, disconnect the lead wire in the trunk (light brown) by the gas filler pipe, The gauge should read way over full, Ground the wire and the gauge should read way under empty, If you still have problems then you know that the tank unit is Okay and that your wiring is messed.
Make sure that the gauge wiring stands alone from all other circuits, Good Luck