wurz154, you are wrong, all the speakers are 4ohm and you can change it easily. The instructions to do it are here:
http://bmwfaq.com/showthread.php?t=88824
Speaker :
http://www.bmwfaq.com/attachment.php...2&d=1121802682
I had change the rear and front speakers and the radio. If you want to see how it's look check this:
http://www.bmwfaq.com/showthread.php?t=105844
is it enough?