Hello Jason. I think I have an idea how your wire burn up. Did you hook the 12v jumper cables directly to the battery under the floor? or to the battery cable connection on the starter switch box stud?
If you hooked up at the top of the starter, on the switch box stud, then it is possible that your jumper cable clamp did not contact the battery cable, but DID contact the smaller wire going from the stud through the firewall then to the ammeter. When you tried to start the car, there would be a lot of amperage going through that wire. I need to think on this again.. Is there any chance you put the 12v jumpers on as on a negative ground car? It’s easy to do, and usually results in a lot of ‘fireworks’.
If your ammeter is not burnt internally and has low resistance, watch the needle when you apply some current through it. if the needle moves, you avoided damage to the meter.
I believe the wiring diagram in the owners manual lists the wire size. Some heavy wire like a #10 and #12 is used. most is #14 and #16, Wire gauge is inverse: larger number is a smaller wire. There is no #18 as I recall.
Hope this helps. Greg Long
Oh, if you DO decide to use a 12v battery to assist starting your car, make the connection of the jumper cables on the battery posts. Then engage the starter, and THEN connect the cables to the 12v battery. I typically will put a 12v battery on the running board next to the open driver’s door, make the connections on the car battery, make one connection on the 12v battery, double, then triple check polarity. Then hit the floor-starter button, and then touch the last cable clamp to the 12v battery with my left hand.
It is not a good idea to leave a 12v battery hooked to a 6v battery, it is a LOT of current and too much voltage and can cause the 6v battery to gas-off or even explode… you do NOT want a shower of battery acid on you or your car.. But when there is a load on the 6v battery and you temporarily hook a 12v battery via a jumper cable to the battery, the 6v battery is safe from my experiences..