My car has the spoiler on it. The first day I took it home, like an idiot, I opened the trunk with it, and it was perfectly solid (still is). I don't know how mine was attached, but it seems very sturdy. Hope this helps!

 

I've already installed the M3 lip on a friends B5, it already came with the two sided tape on side already stuck on the spoiler. The thing barely weighs anything so that 3M Red Tape is like cement, if you DIY make sure you get it right the first time.

First off, we were two people installing it.
- I unpeeled the protective plastic off the 2 sided tape (the side that's still covered as the other side is already stuck on the spoiler) at both ends, just slightly. Both of you align it to have equal spacing between the trunk edge and the spoiler, I would have to estimate around 1/8 inch.

- Stick both sides on the truck, where you want them, best if they're near the very back (obviously). Slowly removing the protective plastic from each side. I notice we had to slightly bend the spoiler in the middle to get the exact contour of the truck.... that's it.

I would also suggest you clean it well, such as in the posting on Audiforum.com.

Cheers,