I had this same problem with a hotrail kinda pick-up (Dragonfire) in my mustang bridge..
Still don't know how to wire it correctly.
BUT with a regular JB humbucker I wired it like this (and still works fine):
- red and white soldered together and taped of
- green were white was
- black were black was
- bare wire to the cup where the bridge goes in
Hope that can help you out a bit...
Here's a good link I found with other wiring possibilities for Mustangs
http://atorih.iobb.net/~dgb/mg_mod_e.htm