All gauges except the ammeter are a simple series circuit, functional path is from ignition switch.........to cluster.........through the instrument voltage limiter.............branches off to feed power to the fuel/ temp/ oil gauge if equipped.........through each gauge..........back to the sender.........to ground
So if the fuel works OK the power/ IVR is OK, that leaves the gauge unit, the sender wire, and the sender.
If the gauge drops with sender disconnected, it's the sender
If not, suspect the sender wire. About all you can do is pull the cluster to access the wire which appears at the cluster connector
