65 Dart Instrument Voltage Regulator Tips

There is no "ground" terminal on the ammeter. It is a through circuit, both sides considered hot.
You probably lost your instrument ground when you pulled the cluster out, the circuits ground through the housing to dashboard to body ground. Try attaching a jumper wire from the cluster housing to a good ground, you'll most likely get your power back to the affected circuits.
Worst case scenario, you may have taken out a fusible link when you tested the ammeter to ground, if you did it when that circuit was live.