Erratic Ammeter

"The actual" problem............is likely a bad VR if it's the original "relay" type, or a bad connection in the post above, or bad alternator. The "functional" circuit path for field wiring is:

Battery.........fuse link.......bulkhead connector..........ammeter........welded splice........ignition switch..........bulkhead connector (dark blue)........and out to the VR

BAD CONNECTIONS IN THE output side of the alternator:

Alternator.........big black wire...........bulkhead connector.........to "welded splice......"..........to ammeter.........out bulkhead connector (big red) to fuse link and batteryl

ALTERNATOR ITSELF: Sometimes vibration can cause the stator windings to rub against themselves and rub the wires raw. Have him run the car "in the dark" and look at the alternator as you run the engine at different RPM and rev it. A "lightning light show" in the windings means "the alternator is done."