The sync will fit into all 2009+ mustangs (and that is all the dealership will install it on if you want the system to be under warrenty). Other than that they told me it will not fit in the other mustangs.
But I have seen people modify the system slightly to get it to fit into their 2005+ mustangs and have seen kits around to do this, I just can't remember off the top of my head which sites I saw them on.
__________________ 2007 4.0 V6 Premium Mustang -- Auto, Vista Blue
Perfomance Mods -- C&L CAI w/ 91 Race Tune (BamaChips), Steeda STB.
Apperance Mods -- 6" Shorty Antenna, Sequential Tails.
Future Mods -- Magnaflow Dual Exhaust Kit, upgrade front grill to GT w/ Fog Lamps | 
Sync for the mustang 
