There is a tan wire in the harness that goes into the trunk. This the wire that goes to the sending unit. Trace the wires to find the correct one. Disconnect the tan wire and ground it to the body somewhere. If the gauge goes to empty the harness, trunk forward, and gauge are ok. Follow the tan wire down to the sending unit and make sure it's connected. Also check the ground wire from the sending unit. I would remove the ground, clean up the connection and reconnect.
It could be the sending unit but if any wire is not connected or you have a bad ground it will read full. The sender connects the gauge to ground, with varying resistance, as the fuel level goes down.
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Hank
70 Chevelle SS396(454), M20, 3.73
My Chevelle
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