Gauges 1966 temp/gas part missing

For what its worth ( and this applies to all of them ) ... Most everything on these cars relies on a chassis ground path. A daisy chain of nuts, bolts, screws, etc... What this means is the instrument panel will not function properly while dangling from the harness connectors unless you attach a ground jumper. The amp gauge would work, the oil light and park light would work. Nothing more. Exception... Some aftermarket solid state regulators can send 12 volts through the thermal gauges when its ground path is broken. For these reasons, To add a perminate dedicated ground wire to the instrument cluster is recommended to any and all. Good luck with it.