You don't need to replace the burned wire, just snip it at both ends and run a new wire. The ammeter probably shorted out, via high resistance and too much current. That happens with ammeters and is why a lot of folks prefer voltmeters. The wire runs from the circuit breaker next to the starter solenoid over to the fuse box. I know you say it doesn't have a fuse in the #1 slot, but the hot side of the fuse is connected to the fuse below it. That's the charging path. If you look, you can see it. Just cut the wire at both ends, tape it off so it looks nice, and replace it with a straight shot of ten gauge wire from the circuit breaker to the #1 fuse position. Then you can just put a voltmeter in the dash. This is not a big deal.
