[dsraven]

here is a pic of what it looks like behind the cowl.

lots of guys close the cowl vent grill in. some simply remove the vent grille and weld a panel over the grille part and then screw the part back on. that way it is easily removed later if you need access for some reason. I will not be using my kick panel vents either so I will likely screw a sheet metal panel over the inside and seal it up. not sure about the cowl vent grill yet. thats a waize down the road.
the other pic is a replacement panel but if you look at the lowest point on the new part you can see a little flap of sheet metal hanging out there. that is usually bent inwards to restrict access to the vent drain hole in the area. check yours for debris by looking up from the bottom side. wear safety glasses.
hope that helps.