I used to have the Thule trunk mount and it worked OK. Anything that's gonna touch the paint will scratch it...doesn't matter how clean your car/bike rack are. And if you have a light color car, the rubber thingy on the bar across the trunk will make a mark on it as well. Also, you have to really tighten down on everything (which is somewhat hard to do sometime), else, you'll risk scratching your car more. So, as you can see, I'm not a fan on the trunk mounts at all. I would recommend finding a Thule lift like the one that BMW offers for the X5 (see the X5 accessories catalog for a picture). The Thule lift is not shown on their website, but I remember seeing one on a printed catalog I had a few months ago. This should make mounting you bike on the roof much easier.