Turning key to "run" causes all power loss.

Go straight to your bulkhead connector, the big fat wire, I think it's black, and start at the fusible link, then the bulkhead, then the ammeter.
I can almost guarantee that's where yur gonna find the problem.
If you need to take the dash out, be sure to disconnect the battery first.
You cannot test the ammeter with an ohmmeter, you gotta pull some amps thru it.
If you decide to bypass the ammeter with a jumper, make sure the jumper you build is up to the task. What I did was put both wires on the same stud. and tighten the dickens out of it.
This left me without a charge indicator, so after every time I wind it up to 7200, I gotta check that the belt is still on, lol. I'm kidding. I mean I did that on the road to get home; then I fixed it.