Volt /Ammeter at odds ??

From your bulkhead pictures, the terminals look like they have the dreaded green corrosion on them. Could be another problem in the future.

There are 2 wires that carry heavy amperage loads. They are both located in the center connector. They are either black/black or black/red heavier wires [#10 or #12]. It looks like one of them was modified at some point. Looks like it was pulled thru the bulkhead connector and reconnected on the engine side. I would do the same thing to the other heavy wire. OR, pull them both out a little and solder them, shrink wrap, and tape. That will eliminate any heat build-up and you'll still have a working ammeter.

I'd disconnect the negative battery terminal, and pull off the wires from the ammeter, Wire brush the ammeter terminals with a steel toothbrush, carefully/lightly snug up the ammeter nuts, sandpaper both sides of the outer terminal nuts, assemble with grease or silicone, and tighten the nuts.

If your alternator and regulator are working as they should, the ammeter normally shows positive at idle without fan or headlights on. This depends on idle speed also. ---good luck--