get the demensions of the box and figure out how much space you have under the amp rack. You will need a mounting location for an amp for the sub as well.
If you remove the factory amps then you wont have any factory speakers, unless you have an aftemarket headunit that can power the door speakers/trunk speakers and tweeters. This is your best option anyways when going with an aftermarket sub.
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)