I have the new filter. I pulled off the panel behind the glove box on the passenger side. There doesnt seem to be anything there to replace. There is (much to my surprise) a large styrofoam block of some sort that takes up a majority of the space.

 

That's because the cabin air filter is under the hood. Pop the hood and look just over your battery on the passenger side near the firewall and you will see a black plastic lid with three clips on it. Lift the clips up and then lift that lid up and out and you will see where the filter goes to filter the air in the cabin. I have a 2005 C230, so I'm guessing it's the same on your car. Your filter, if like mine, is a pleated white paper filter approximately 8" X 14" and angled at one end. Right? It goes under the hood. Hope this helps.