There is a drain in the upper left and right hand side under your hood up by the windshield. If they are backed up with leaves and debris the water can back up into the interior of your car. IAMFINK knows how to clean these out, I'd suggest that you pm him. This might be where your problem is coming from. A few weeks ago we were discussing this problem with an SLK owner and he had some screens made up that cover those vents and prevent debris from going down in them.