On my truck it was the ground on the sending unit inside the tank. I removed the S.U. from the tank, turning the S.U. upside down, there is a screw that holds the plug to the base of the unit. I removed the screw, which separates the plug from the base from the S.U. itself. Between the unit and the base was all rusty. I cleaned the surfaces up and put every thing back together. And now I have a working fuel gauge.
F.Y.I. I did clean the grounds on the outside of the tank before resorting to this procedure.