How rare is electric rear defrost?

FC7Demon340 said
No problem and yup easy to do. Those big wires connected to the 2 terminals on the back of the gauges. The dash harness had the regular ground that's usually attached to the gauges attached with them. But the positive on the dash harness was capped off with one of those factory style clear plug caps.

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.