Same thing happened to me this past Spring. Soaked the 3 screws with Liquid Wrench, managed to get 2 of the 3 to loosen up and come off, but the third wouldn't budge. So I drove to the dealership and asked if they could help. They got it loose. When I got home and finally removed the cap, I found that the plastic "ear" that houses the 3rd screw hole had cracked and was just hanging loose. I don't think the dealer did it -- I'll bet the screw was on so tight, it cracked the housing.
Anyway, you might try getting your local dealer to help. If they bung it up, at least the car's in the right spot to be repaired.
1989 190E 2.6