To decide whether the trouble is towards the sender or back towards the guage: remove the sender wire (the one coming from the guage) at the tank and let it dangle in mid air, turn the key on, guage should read full. If it does - the guage is good and trouble is in the sender. If it doesn't, trace that lead back towards the guage, it's grounded somewhere.
Note: make sure the sender wire to the guage is connected to the sender post, not the ground post!!
