Fuel gauge Intermittently worked,..now nothing. 66 Valiant

Just incase ... Please know the amp gauge is red hot at all times. Remove the neg' battery cable before going at the panel removal.
Most will attach the wiring and attempt to test the panel before completing the reinstall. It wont work without its chassis ground path. That is though the screws that attach it to the dash ( like most everything else on these vehicles ). Adding a actual wire for ground is recommended. Good luck with it.