Guage says full.....it's NOT!

My fuel guage always goes to FULL with the key on. I know it isn't. I've replaced the sending unit in the tank and the guage from a donor car. Is there a relay that would be JACKED UP causing this? And where would it be?
95 Grand Am:screwy:

Anyone? Beuller.....Farris Beuller....

Sounds like a short to me. Unplug the whole mess at the fuel tank, turn it on and see what the gauge does. If it still shows full, start tracing wires. If it goes down to empty its the sender.