I'm no expert in this area, or this vehicle, but it seems to me that if
an alternator checks out OK (that's a BIG "if"), and the battery has
been changed (and goes dead, indicating the discharge shown by the
ammeter is correct), then the only other possibilities are the wiring &
connectors for the charging and sensing wires from alternator to
battery, or the grounding wire from the alternator's internal
regulator... or a horrific short, which is unlikely or you'd see a
constant charge on the ammeter, not a discharge. I'd believe the
ammeter, which reads the flow of current, and assume the alternator is
bad, so I'd start by removing the alternator and having it bench-checked
by a professional alternator rebuilding service.