instrument problem

You grounded sending wire at sending unit and fuel gauge does not work?

Move up to sending wire at kick panel connector for rear harness, and ground it there

Next, loosen/ tighten the nuts on the rear of the panel

If that does not do it, check the pins on the harness connector at the PC board connector.

By now you have pulled the cluster.

Put a 3V battery directly across the gauge unit. If no reading, it's the gauge unit. (That's two 1 1.5 batteries in series)