fuse box

The fuel tank has nothing at all to do with this test. THINK about when you tested the temp gauge

1...you removed the wire from the sender

2...You used a jumper wire or clip lead to connect the sender wire to ground

3....You turned the key to "run" and watched the gauge.

You are doing the very same thing now. You are going to ground the WIRE in the harness which was connected to the fuel gauge sender