Connections at the back of the ammeter along with the bulkhead connector are known weak spots in the system... since day 2.

If it is a resto and/or you really want to keep it "stock" then 1) make sure electrical loads are not exceeding OE. No big stereos, alternators or other accessories. 2) replace failed wires and connectors 3) scrub remaining connectors clean with emory or replace every connector. 4) grease every connector and re-assemble 5) enjoy.

Otherwise, bypass the whole (weak) affair. A 10 gauge or heavier wire between the alternator post and batt+ or fender relay will take the heavy current off the bulkhead connector and ammeter. You will need a voltmeter on a switched ignition circuit to monitor charge system condition. I had my factory ammeter converted to volt meter. No visual difference. The bypass wire should have a fusible link sized 2 gauge sizes less than the main wire... or fuse sized at alternator output +25% (minimum).

I took the bypass approach.