The white wire coming from the alternator goes to what? It is unhooked and the charging system is not working. I am sure the white wire not being hooked up is the problem. Nobady seems to know where it goes.
You don't list a year of mustang, nor the generation of alternator being used.
There is no white wire. If you meant white/blk, it's part of the stator circuit.
If you have an aftermarket or modified regulator wiring harness (or a '94+) and the white/blk wire has a female spade terminal on it, it would plug into the side of a 3G alternator case.