Tony,
A FULL gauge reading means a "open" between powered gauge and ground supplied by sender. You say you replaced the wire from gauge to fuse block but how about the wire from fuse block to sender. If this wire is 'open' the gauge will read full constantly.
The trick to prove the gauge is to turn the key on and using a small jumper, ground the wire in the fuse block to a clean good spot...the gauge should jump to EMPTY. That connection in the fuse block is not powered, it's just a connection for the wire back to the tank.
Stuart
