when i was doing spring cleaning last year I took off my wiper cowl and noticed quite a bit of tree debris so being the smart guy that I am I took my water hose and shoved it deep into the frame. meanwhile i can see dirty water and pine needles flowing out from behind the fender/frame. I cant help but smile and think "look at me go, you'll be clean soon enough ol girl!" then out of the corner of my eye i see a tsunami wave in the passenger side floor board..... Efff!
well needless to say, 4 days later it got musty and mold smell so I pulled the carpets, pressure washed them and reinstalled after a couple days of air drying.
long story short is that's where air comes in but in theory it should flow out from behind the fender/frame but after time debris will clog the flow and back it up like a toilet after the county chili feed. i would recommend taking off the wiper arms, cowl and inspect for leaves, mud, pine needles.
hope this helps... D