I think I follow what your describing.
The ammeter is seperate from the rest of the gages. It's the only gage that connects directly into a wire path. Electricity flows through it for the charging and when things are running on battery power it flows the other way.
The black wire on one side is not ground (even though its black) - it connects to the main splice.
The red wire that was capped off would have been the original connection to the battery. Apparenlty easier for the factory to to cap rather than remove it from the bullkhead connector.