How many wires have you got hanging from the loom that will reach the sender terminal, and what colour(s) are they?
If it's been butchered and there's no wire at all (i.e. you can't identify any spares in the loom that are the same colour as the one connected to the gauge) you may have to run a new wire from the sender to the dash.
Check google for any posts by 'John Strenk' about troubleshooting CJ gauges. He's got some great info and technical tips.
These 2 links may have some good info that could be useful...
http://www.4x4wire.com/jeep/tech/electrical/gauges/
http://civilianjeep.info/Strenk/Gaug...QuickFacts.htm