Chasing a parasitic drain on the ammeter circuit

Might be good to "back away" for a minute.

Don't eliminate stuff by "logic" because something could have a problem and be part of the problem. Loose contacts walking around in the ignition/ wiper/ headlight switches.

If you can see it on a sensitive lamp, that might be better than a meter, because you can "watch" it out of the corner of your eye.

So just to repeat:

1...Pull all fuses. If the load is still there, move on
2... Think and LOOK for hacks, like alarm circuits or aftermarket audio gear
3...Disconnect all you can to strip away, simplify the problem. As I said, EG separate the tail harness.
Pull out the headlight switch. And for SURE pull the ign switch.
4....Get the diagram and see what all comes off the ammeter circuit. Not all that much, internally. It feeds the fuse panel hot buss. Go get the fuse panel LOOK at the hot buss, and make damn sure nothing has been spliced in there

Ammeter circuit (welded splice) also feeds the alternator, which should now be disconnected and the black taped off, some years feeds the wiper switch (park circuit is live!!!) feeds the headlight switch which should now be disconnected.

We are running out of options.

As I said, separate the bulkhead connector. Does it go away?

Then if so, jumper power from the battery into the big red terminal of the bulkhead. With everything (above) you have disconnected, there cannot be much in there. I don't know how the ammeter could leak to ground, but you never know.