By: Jim (Jim O. from BimmerForums.com)
1. Rear seatback latches - clean and lube
2. Rear seats - condition the leather
3. Rear seatbacks against the parcel shelf - try putting a strip of tape where they rub. Or, a piece of stick-on foam weatherstripping.
4. Rear parcel shelf - remove, shim with some tape or foam weatherstripping, then replace
5. Catches on the side pieces next to the rear seatbacks - remove, clean, lube, replace
6. Rear seatback pivots - clean and lube
7. Rubber weatherstripping around the windows - clean and gummiphlege
While you're at it, clean and lube the door latches, and the trunk latch.
Try that, and see what happens...the best thing to do is to add all the "clean and lubes" and the leather conditioning to your normal maintenance routine.
~Jim