I saw a C series this morning as i was going home and saw some slots in the rear lights. I am guessing they are some kind of synthetic jet or something and help reduce the wake. Here is a link to a mercedes site in Ireland. When you rotate the picture of the car you should be able to see what i'm talking about.
Mercedes-Benz Ireland - C-Class - C-Class Saloon
It's a big page so hang in there!
ollie