1996 fuel gauge problems

[tforbes]

I will start by saying thanks in advance for any advice. I recently bought a 1996 3500 GMC Sierra 4x4 CC with a 454. When I went to pick it up the guy said the fuel gauge didn't work. No big deal, this is my 5th GM truck and I have changed the sender before. The screwy thing is, the gauge stays WAAAAYY past full regardless of the level of fuel. It seems to still move in correlation with the level of fuel but it is always way past full. Upon further inspection of the tank, bed, etc I noticed several holes that make me believe one of the previous owners had an external tank mounted in the bed and possibly spliced into the wiring harness for the gauge. Consequently, it may not be hooked back up right. I don't have a problem replacing the unit, but if it is in the wiring I may not need to. The gauge acts as though the float is stuck, is that possible? The gauge reacts when you turn the key on and off as though it is working properly just getting a false signal. I don't mind running off the odometer, but $100.00+ per tank is getting old. It would be nice to just drop $50.00 in there every once and a while and use my fuel gauge. Any ideas??

Thanks again for any and all help in advance.