Now I understand the problem. I went out and checked my 92 300E and I believe it has the same rivets that you describe. I tried my trim removal tool. It looks like a plastic tie rod fork. I was able to pop the rivets out of the metal by wedging it between the metal and the rivet head. You might want to wait for a confirmation on this considering the year difference between are cars but I think that's how they come out.

Sorry I couldn't be more help.
Mike