I have a '95 C280 and have noticed that occasionally I get venting air through the side air vents and through the top of the dashboard (where you would have defrost) when the climate control is shut off. I thought when it is shut off, no outside air should be entering the car. Is there some flap that's not closing somewhere?

 

sounds like you have a recirculation flap that's leaking venting outside air in. on c classes the only flap that i have ever seen is that one flap and i personally have changed about 20 of them the flap is like 30 bucks i think. however the dash has to come out. unless your really good and have a really skinny arm and can snake it out by removing the lower under dash panel and the glove box. but i don't recommend this method its only for trained professionals.

 

the entire dash has to come out? You mean it's a thirty dollar part that cost $1000 to install or more?