Have you checked if that terminal is shorted to ground yet?
A single wire would just need whatever is on the other end to be fed 12v switched power from that side - typically a double wire on the voltage regulator instead. But your harness may already be doing this.
Yah the terminal was loose on alternator,l tighten it back up. That would cause it short out and melt that wire? Ok l can eliminate the double blue wire and make it a single wire then?