The fusebox and the headlights are supposed to be wired between the alternator and the ammeter,
That's what the picture shows here. Battery becoming very hot, post 37
But the ammeter was acting like it was in line with alternator output.
When it didn't show discharge with the engine off, lights and heater on, that confirmed it.
Here's one way the fusebox and headlights could have ended up on the wrong side of the ammeter.
Go back to the thread to see what actually seems to have been done.