No power.. Ammeter questions.

Is this from a 73 Dart? Please post your car info, I can look up the wiring schematics.
Also, did you at times turn on the ignition key past the accessory (no engine running) position for a long period of time? In that position, the ammeter draws a lot of current that otherwise would be absorbed thru the charging system when the engine is running, which could burn the contacts or the meter (and can fuse points in the distributor if not electronic ignition.. been there, done that)
When you bypassed the meter to get car running, did you plug the connector back together? The connector may power other components, like the radio, signals...

The wiring schematic will give you many answers.

I suggest rebuilding the meter printed circuit board and rebuild/replace the ammeter, there are companies that rebuild the meter clusters.