underdrive pullies are pointless, its almost 200-300 bucks +install for 2-3 hp and more strain on the electrical system. Also for aux you just need an in line converter to go to RCA then that into aux, honestly though your probably better off getting a new headunit, you can keep the speakers just get a new headunit with aux, usb, bluetooth, etc.
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)