I am going to be purchasing a 1 series BMW with the upgraded sound system "Premium hi-fi." The car comes equipped with a total of 10 speakers of which I believe include 4 tweeters and two subwoofers. The subs are mounted under the front seats and by the looks of them I believe they are 8 inches each. I was wonderign If anyone had some idea as to whether or not I could put another sub in the back... I was thinking something in the 14 inch range... Im not sure if the amplifier is in the head unit or not and I guess my question is whether this would become some huge project requiring me to rework my whole system just to add another sub... thanks in advance for the advice

I suggest you post this question on one of the dedicated BMW forums - you're likely to get good answers from dedicated BMW enthusiasts. Check out:

Car forums and chat rooms for the automotive car community. Car Message boards. Car Community, Car Social Network, Automotive Social Network (http://www.roadfly.com/forums/)
Bimmerforums.com - The Ultimate BMW Forum (http://www.bimmerforums.com/)
Bimmerfest: BMW Forum, BMW Research, BMW European Delivery, 335, M3 (http://www.bimmerfest.com/)