I just did a retrofit of the Bluetooth and I'm pretty familiar with what goes into installing oem bluetooth. What you'll need to replace will depend upon if you have BMW Assist (you will know if you have this feature if there is an SOS button in your overhead console in between the two map lights)
If you don't have an SOS button (this button should be in between the two map lights in the overhead console) this is what you'll need to replace http://www.e90post.com/forums/showthread.php?t=783895
If you do have an SOS button, this is what you'll need to replace http://www.e90post.com/forums/showthread.php?t=599310
The unit (either one) is located inside the trunk underneath the trunk floor near the seatbacks of the rear seats.