[VetteVet]

If your gauge reads full hot with the key off then your gauge May be bad. If your sender wire is pinched and grounded, it will read continuity end to end even through the ground.
you should unplug the cluster plug and if the wire is shorted to ground then the gauge will return to cold unless the short is in the cluster body. Some of these gauges will stay the same when the power is off as they read while the key was on. The needle on the temp gauge would stay where it's at until the ground is removed. That would explain why it reads past hot all the time.