After a fire.....
No, actually the tailgate switch and wire are OK, just seems like water got in it, and it no longer turns and springs back.
Electriaclly, it functions. Looks like the fire began at the bulkhead disconnect. Anyone else suggest if the ammeter could lead to the bulkhead burning up?
Lets answer in reverse order because you can help answer these questions better than us since you were there.
In a factory harness, the ammeter only carries current when the battery is charging, or when running the car on the battery (such as when the alternator is dead). Normally the ammeter needle is centered, indicating zero current in or out of the battery.
So review what was going on when the fire started. What was the scenario? That will be one set of clues.
Heat is created when current runs through resistance. More current or more resistance will generate more heat.
Look at the wires connecting to the different components, especially at the connections. Most of the ammeters consist of a metal bar with two studs pressed in, and an insulator. You'll see if there was heat in it, or outside of it, or not even close.
There's a couple ways an electrical fire can start. One would be from normal use of a circuit with a poor connection that got so hot it ignited something flammable nearby. The other is from a short in a circuit. A short is an additional or easier path to ground.
If the tailgate switch had water that was causing a short, that could increase the current.
If there was any wire that insulation had rubbed off and was touching other wires, or worse, ground, that would cause a short.
A fusible link, which a '64 probably had, helps protect against a direct short from battery to ground in the main circuit.
Fusible Links in Charging Systems with Ammeter
