Had this problem, yesterday I took the whole wiper cowl off, and took out the cabin filter. Poured a cup of water over the housing, sure enough niagra falls on the inside. took out the filter housing, and found the gasket corner was gone. I went to menards and bought a $2 roll of foam weatherstrip, sticky on one side. I found it near the winow/door section. I made a new gasket (you need to remove backer as you go along or it wont bend for corners) on the back of the housing, then overlapped with another layer, putting the seam in the opposing corner. I probably didn't need to overlap, but I figured it couldn't hurt in case things expand/contract. Bolted it back down, poured water over it again, all down the sides and out the corners to under the car, inside the housing was dry. Not a bad fix for $2!