Just out of curiosity, have you turned on the headlights and some other accessories with just the battery to see if the alt needle
moves at all? One issue with the 1976+ Ammeters is that if the shunt wiring has a break or is corroded, the amm will no longer read...
but will allow the vehicle to continue running and the driver is none the oblivious until he gets dim headlights and ignition misses, since
the driver just assumes that the charge/drain is evened out by the alt... the main benefit is that out weighs this is the car will
still run if the amm goes poof unlike the Pre-76 vehicles...

Yikes! Wonder how that happened.