Under dash short

"connection fried at fire wall".
I think you answered your own question.This is typical. This particular connection can and often does develop a high resistance to the throughput. That makes heat, smoke,and fire.
Disconnect the battery,Isolate the circuit, Ohm it out.
Since the engine quit, that main powerfeed was interupted.There is,between the firewall and the ignition switch, in the harness, a welded splice. Power branches off from here in at least 4 different directions.Power branches off to 1)the IgSw, 2)the headlamp sw,3) the fusebox,and 4)the horn relay, there may be more.None of these are fused.EDIT; but then you knew that(doh)

Question is; why didnt the fusible link do its job?You do have an F-link, right?

The ammeter is mounted in plastic and is a dead short across the terminals. It is built that way.Inside the ammeter is a short bar across the studs. A very fine insulated wire is wrapped around this bar which, inductively, powers up the needle.
However,that device is mounted very near to the metal dash superstructure. Its difficult, but not impossible, for me to imagine a stud to somehow short to chassis ground there. Poking your head in there, should find it obvious. Those studs are a convenient place (for the factory at least) to tap power off, for other things;such as a convertible top sw(circuit-breakered), or a clock(unfused).
I usually take the seat out. Its waaay more comfortable that way.

I used to carry a long jumperwire, in the gbox.Long enough to jump from the battery to the ballast resistor.And a mini toolkit in the trunk, with something to short the start relay. After that connection left me stranded the first time, I used those items to get me home. Then I cut that wire out and replaced it with one piece, from end to end, and with a new F-link. No more problems.