Don't get mad at me if this is wrong, but I replaced the jack pad on my C55, and I seem to recall when doing some research that basically all Mercedes SLK and C class models used the same part.

You can buy it here for dirt cheap!
Search - RM European Auto Parts

Again, I can't guarantee it will fit but honestly for the price I would buy one just to check. Very easy to replace. All it is is a fat black plastic plug that screws into the frame with a flathead screwdriver.

I have purchased from rmeuropean a few times and have always been pleased with relatively quick shipping.