Water Temperature Gauge Not Working

I think I follow. I fully reinstalled the cluster just like I took it out. I will take it back out and trace all the mounts to see if I am missing something. I don't notice anything that seemed out of place but I don't know for sure. Maybe I will try running a direct ground between the cluster and the body and see if that makes a difference. The previous owner had obviously been messing around under the dash because he installed an aftermarket radio.

A permanent dedicated ground wire is strongly recommended. Original daisy chain of contact points might have been fine when everything was new but... it never equaled an actual copper wire as a conductor.
45 years later you could take the time to clean every contact surface. Several owners have mistakenly painted over many of those original contact points.