Hiya Filth!
Disconnect sender unit wire, see if the guage goes to zero. If not, ground wire to something of negative polarity (engine block), does it now go to zero? Check that engine block has a good negative connection. Measure voltage at sender unit wire, I would think it should be 12 volts. If this does not help I think you will have to pull the guage out. Give us a call if you want. Good luck mate.:drink