Help with fuel gauge

[R.O.B.]

My fuel gauge was working just 2 days ago, now it just reads empty when it should be over 1/4 tank.

Back story, I inherited this 67 C20 a few months back, and it had been sitting for over 19 years outside. Since I've brought it home, I've installed a new tank and sending unit. Gauge didn't work then. I took out the dash and cleaned up the connectors and the circuit board wiring and the fuel gauge came to life. It had been working just fine until 2 days ago.

If I unplug the wire from the fuse panel (with key on) the gauge will go to 3 o'clock. Plug it back in, empty. If I ground the fuse panel connector, empty. I even added 5 gallons of fuel, no change.

Please give me some direction on where to go with this, as I ASSUME it's not the sending unit or the float being as they are only 2 months old.

Rob