He (your mechanic) is full of crap. Re-read my last posts. On an otherwise normally working car, you can pull the cluster out and set it outside in a mud puddle. Then bolt the red/ black ammeter wires together and start it up and drive it.
You need to please start reading my posts more closely. I am not bragging here, I know how this stuff works and I know how these cars work. Most/ all these mechanics may be good at what they do, but they generally are not familiar with these old girls
There are three or four major points that you likely lost power........the wiring to and from the fuse link AND THE BULKHEAD CONNECTOR TERMINALS for the big red and big black wires, and possibly the WELDED SPLICE which is a few inches down the big lack wire starting from the ammeter black eyelet connections. Unwrap that harness a few inches until you find it, unwrap it and inspect. Also suspect the ammeter eyelet terminals themselves.
The power coming into the interior from the battery is the RED ammeter wire, and the feed off to stuff like dome lights and the ignition switch and headlights is branched off from the welded splice LOOK at the simplified diagram I posted. that well illustrates the setup
