Field wire (12V key on) melted

[Warlock Wanted]

Turing a tuning session on my 78 LRT I noticed the wiring harness burnt through the tape, under closer observation the field wire feeding the alternator was completely melted through the casing and the tape of the harness. Thank goodness no other wires were damaged or the truck didn't burn to the ground. So I am replacing the wire all the way back to the bulkhead connector but I am wondering what shorted out this wire? Nothing obvious in the harness so wondering about the alternator?? Could that be the cause of the short? Is there any checks I should be doing before putting it back together?

Thanks in advance for any help.

 

[Mr.Mopower]

Research "ammeter bypass"

 

[Warlock Wanted]

Yes I am planning to add the shunt wire, but this is on the feed wire to the alt not the output. It is key on 12V but obviously it went to a dead short (ground) to burn it the entire length of the lead. Just not sure how. Thanks