Like the others have said, troubleshoot to figure out the source of the problem... Someone mentioned it before, but I will again anyhow, ground out the wire back at the sending unit, this should peg the gauge to full. If it does, the sender is the problem, you can either repair or replace it; I replaced mine. HOWEVER, if this doesn't peg the gauge, there are a few concievablepossibilities: the wire going between the sender could have a short in it, pull the gauge panel (you're gonna have to do this anyhow w/ the other possibilities) and test for conitinuity between the - post on the gauge and the wire back at the sender. If you have conitinuity, then there is something wrong on the + side. At the + post of the gauge, depending on the quality of your multimeter, you should see approximately 5 volts; if you have a good meter (I have a few, my best meter is a Fluke, and it does this, my cheaper ones don't), it will not read steady because the voltage regulator is opening and closing. If you read ~5V at the gauge, the gauge is bad. If not, the VR is bad, or there is a bad spot in the circuit betweent the VR and the gauge or the VR and the +... Good luck!
-Jason
'72 Mach 1 || '90 Talon TSi AWD
http://nosaj122081.tripod.com/parade3.jpg
"Thats not a leak, my car's marking its territory!"
"If you've done it, it ain't braggin'." -Roy Rogers