Try changing out the cabin filters with new OEM units. When I got my 2013 X5d it didn't have a bad smell but it was definitely not the "stock smell" or whatever. I changed out the cabin filters and within a day or so it is back to what I would consider a stock smell. Just a faint smell that is different than the air outside, that's about it.

I've also read in a lot of restoration forums that if you go buy a piece of upholstery vinyl from a fabric shop and stick a chunk under each seat during the summer it will restore "new car smell". I've never tried it but it would make sense.