Noticed when driving top down / windows down the passenger seat belt flaps around and makes an annoying noise. Didn't want to keep it fastened all the time, and the BMW OEM hanger where it connects to the head rest made getting the dog in and out of the back more challenging. Our 2006 had clips for the back belts that got me thinking about a similar solution. After confirming BMW doesn't make anything that would be perfect, bought this on Amazon for $5 - meant to attach to a cell phone to hang on your belt.
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
Perfect size for what I needed. Tried using the attached double back tape but didn't help much. Instead I put a small amount of crazy glue on the back portion of the clip, inside so that it would adhere to the trim. I pulled the trim up, slid it back there while holding it open so as not to get glue on the front, and let it set. One thing I would do differently next time would be use thicker gel glue, the very thin liquid got a bit messy but did the job. Anyway for $5 figured this was worth a shot, worked very well. If you ever want to remove you'd just have a glue mark inside the interior trim that you'd never see. Just need to scrape a little glue mess I made off the clip, but overall very happy with the result.