the trunk part is easy to take care of, just spray a bit of silicon into your latch compartment on the rear gate, that is going to help for year or two...

Seat squeek...
 

I recently took my '03 4.6L in to the dealer for a bunch of things and one of the items on my list was a squeeky driver's seat. My seat was squeeking each time I entered/exited the X5 or took a sharp corner. Quite annoying! I was (am) still under warranty and the repair was free of charge, obviously. They found that some of the seat bracket parts were worn and broken. I received an entirely new undercarriage bracket including all of the seat motors. The cost of this bracket was over $800. I don't think that BMW sells replacement parts for small seat hardware involved with the seat bracket, so you might be better off trying to fix it yourself, after you find the cause. Unless you are also still under warranty? Best of luck!