The cap is a little tricky to take off, but not that bad. You need to get a long skinny screw driver and move the mirror to one side so it will give you the biggest gap between the mirror and the plastic. There are three (if I remember right) plastic pressure locks with hooks (for lack of better words). You need to push them together the best you can, so they will slide out of the rectangular space. I am sure someone can explain it better, but I just poked around until I got it off. I have taken them off three times already...and it just takes a little patience to get 1. get behind the mirror 2. to get the two plastic clips together enough so they slide out the hole.
If you are talking about the black plastic piece on the quarter panel...yes it does come off, however, My one clip broke when I took one off, but the other is fine, so I guess it depends on how easy they come off for you...I would chance it and maybe shave them or if they do break they shouldn't be too expensive to replace.
Hope it helped...
BRian