Mmm, it has been wet a lot lately.I think I have another cluster I can borrow a gauge from, sounds like the easy place to start.
That would be my guess as well.. I can;t remember off hand if they had a separate ground wire for the sender, or if it just grounded though the tank and such... I know most of my old Fords just grounded though the tank.. Nightmarish for functioning gas gauges till I ran a separate grounds..