there is also the cabin air dust filter which is housed in a panel below the glove box within the front passenger's foot well. i recently replaced the one in my S420 and there was a very noticable increase in air volume - even more so if you take the old dirty crusty filter out and don't put the new one in right away - even more air!

...something for you to look in to as it's 'only' a $20 filter and it probably needs to be replaced if it never has been.

mike b.