Hey

i own a 2013 c300 and i have been trying to remove the rear diffuser following the threads on the forum but i faced a problem since on the side of the diffuser there are supposed to be torx-20 screws but mine are different
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they do not even look like SCREWS

 

Those are more than likely rivets, you will need to drill them out.

 

They are rivets. You need to drill them out, and then once you install the new diffuser you need to also install 2 new bolts and nuts to hold it into place. You can get the small bolts/nuts at Home Depot or Lowe's for $1 or so.