I have mounted Mercedes emblems two ways. For the grille badge that I mounted on the fuel door, I had read the threads about the Brabus badges falling off, so to make it extra secure, I drilled the door and used epoxy to glue the stem from the inside.
For the trunk badge that I mounted on the glove box, I used a dremel tool to grind off the stem, then used 3M VHB Heavy Duty Mounting Tape (#5952). I bought it directly from 3M's website, but I read on one of the posts here that you can get a similar product from Napa auto parts.

VHB Heavy Duty Mounting Tape 5952