Here is a nice write up on a aftermarket unit. Just watch out for his axxess instructions as they are backwards. Alpine IVA-W205 & Lorex SG4933R rear camera install (w/pics) - The Mustang Source - Ford Mustang Forums
I just installed a 2500 in mine last week. You can twist the 2 blue white together with the pioneer harness blue/white.
2007 Vista Blue GT Convertible 5 spd, leather, Pioneer HU with back up cam, FR500S axle back