Sorry mate, but you trying to install the spoiler right? not remove one?
If you're trying to install, you can always spend a tenner and buy a mounting kit from your local dealer (last time I check was last year and they were available then) I believe the kit comes with mounting plates, protective film, and sealant for your screw placements. This should make for a trouble free installation, but you stillll have to drill holes in your boot lid.
But if your trying to install without drilling and screwing it in, I don't believe your going to get a secure mount with any off the shelf adhesive, and it will eventually come off.
Hope that helps mate
