Skimming back through your posts
@canajien this is a modified '71 Duster.
Can't help with the mods but can get you on a footing to understand the factory system.
Old school underdash gage sets often included an ammeter, but not common now, and you certainly don't need or want two.
I find it helpful to recognize the car has two power sources; battery and alternator.
Each has an output line that is tied together with the main circuits at a welded splice.
The splice is wrapped in the interior harness. On the diagram below it represented by a solid circle. (The '71 Service manual shows splices as diamonds.)
View attachment 1716262524
Everything connected to that splice is connected to the battery positive. So if the battery is connected, all of those wires are hot!
It doesn't matter if the key is off. They are still hot. This why the horn, the brake lights, the dome light, etc will work with the key off.
Ammeter: The ammeter is in the battery feed. The needle deflects when electric current flows past the needle. So it shows electrons moving. It will show disharge during starting and charging after starting. Normally its in the middle since the alternator supplies power when the engine is running.
Fuse box: Some of the fuses are always hot. Others are only hot when the key is in run or accessory position. There is one additional fuse for the instrument lighting. We can come back to that later if you want your new gage lamps to be dimmable.
Seeing the condition of the harnesses in your car, plan on rewrapping and supporting the wire bundles like the factory. Loose wires will scrape insulation, stress and break at terminals, and can cause accidental grounding. Notice a lot of the wiring is not protected by fuses or circuit breakers! The only protection for the main circuits is the fusible link - which may or may not melt before damage is done elsewhere.
