Under the hood (under the windshield wiper arm to be precise) these two filters (different from your regular engine air filters) need to be changed every 15-20k or so. They basically filter the air entering the cabin when you run your air circulation. for the long run, this is what you will need to replace. call any MB dealer and they can give you the part number and cost estimate. you could buy them much cheaper here at the parts shop. Or at
www.************************. Also search this forum for air cabin filter for more details. Good luck!