theres a temp sensor, I'm clueless as to where its at but I bet you can find it in:
http://www.binghamtonlife.com/manual/2gsm/default.htm
If everything is plugged in and you replace the temp sensor and its still now working you may have a bad gauge. On old stuff you could take out the cluster and trade down the leads that go to the temp sensor and hook one to the battery and ground out the other and if it worked the hand would start rising (remove power quickly or you'll burn up the guage). But like I said that was on old 70's stuff dont know if it would work on yours.