15V

That's one - maybe. Sure looks like the heat was at the wire crimps into the wires.
"There's an extra wire added to that ammeter. Wonder if that modification had anything to do with the fire?"
Sure does look like that's a possibility. Very unlikely the factory added a second red wire.
Dude I dont know what your problem is the fire issue with ammeters has been around for decades and I have witnessed it first hand in a 70 Roadrunner. If you dont believe it so be it.
I have better things to do, I'm done.
Byyyyyeeee!