The oem wiring would not be beefy enough to support a large aftermarket set up consisting of a multi amp system. You would need to run new power, ground, add a distribution block, and would need a line level converter to convert to RCA's for an aftermarket amp. If using oem speakers with an aftermarket amp, make sure your new amp has a decent crossover network. If replacing be sure to purchase seperates with supplied crossovers as BMW's lend themselves well to such a system.
If none of this makes sense, take your ride to a reputable install shop. Not trying to be a jerk by any means, I just deal with eager folks with a little less know-how than necessary everyday, and it almost always spells blown amp, sub, or much worse electical damage to the auto.