"Aftermarket" Radios are ones you buy from a store or the like. So if you still have the org. radio in your BMW, then it is not "aftermarket". If you would like to add a sub, box, and amp to the car, I would suggest going to a store like Best Buy, Circuit City, Tweeter, some thing like that, or a place you would trust and HAS
good knowage about your BMW. So don't get the kid next door to put your equipment in.
But you will need a amp, sub speaker, box to fit your car, and a wiring kit to wire to the battey, ground it, and to a line remote, also if you don't have a aftermarket radio you will need a speaker tap box and RCA cables for to go from the amp to the tap box, and speaker wire to go from the amp to the sub.
I hope this helps.