Get a multi meter and measure resistance from the sender at the tank to the sender tube.
You shoud read between 10 and 70 ohms give or take.
Next measure same but between sender at tank to body ground should read same or there abouts.
Next do same at sender wire at back of gauges to body ground.
Next you can VERY temporarily ground the sender wire at the tank and have someone else look for gauges movement
then report back
