behind the gauge cluster, the copper bars that attach the fuel and temp gauge is usually the problem. They are actually resistors to step down the voltage. Complicated damn things to me, but simple in design...I have a friend get mine working. He specializes in 1972-1975 CJ5s and CJ6s, so I lucked out. My gas gauge is a little off, but no more than 1/8 of a tank incorrect.
1976 and newer are close to the same as mine...Need to talk to an old jeep mechanic, I hate electrical stuff....lol