the mach system doesnt take kindly to aftermarket subs/speakers, a head unit with the proper connections should work. I would check with an audio shop or two as well as best buy just to make sure though, but from what I understand you can use an aftermarket head unit as long as you use the correct adaptors/wiring harnesses.
2003 Redfire V6,
Show: Smoked everything, cobra front bumper, gt rear bumper, rear decklid and lower valence blackout with black mustang letters, 5% rear front 15%, black 98 Cobra rims front, 95 Cobra R rear black with chrome lip, black scoops and spoiler and exhaust tips
Go: CAI, True Duals with flowmaster super 44's, Bama Tune, full suspension (lowered, 8.8, 4.10s, UCA, LCA, springs, CC plates)