I just installed an underdash main wiring harness. I have a wire that I am not sure what it is used for. It is a blue wire with a black stripe and it has a ring terminal on the end. The wiring harness is a reproduction bought from Mustangs Unlimited. The original in the car did not have a similar wire. I have looked at wiring diagrams and schematics with no luck so if anyone has installed one of these and can help,I would appreciate it.

 

That's probably the hot lead for the fog lamp circuit breaker. The lead in you old harness was taped back so that's why you never saw it. Tape it back if you're not going to install fog lamps.
Jim

 

Great, thanks for the quick answer.