Brand new 2 months old 100amp powermaster alternator bad?

What was the voltage reading?
How long have you run the car sice you noticed this?
You may have just somehow killed the voltage meter. Or it could have been a freik coincidence.

I don't remember which wire it is, but there is a wire that essentially turns on the alternator. Maybe it is bad, so the alternator is not charging. I had that happen once. The wire became disconnected behind the dash, so my alternator stopped charging. It would test fine, just not charge. So, try looking for that wire.
Check out tmoss' info site. It has the wiring diagram you need.
http://www.veryuseful.com/mustang/tech/engine/
If you find out what happened, post it.