The cowl (area under the wipers) is directly under the forward part of the dash near where it meets the windshield. This area is meant to allow water to drain from the windshield/wiper area. The drains for it are on the upper part of the firewall in the engine compartment and you can see little rubber flappers covering them. If the cowl area is full of junk (leaves, pine needles, etc) as it can sometimes get, it may not be allowing the water to drain from it completely. The best way to see inside it is to remove the wiper arms, then remove the plastic piece which incorporates the screen. You may also want to make sure there are screens covering the oblong holes into the cowl at the outer top edges of the hood (visible near the hood hinges when the hood is open).
I don't know if this is your problem, but it would be a good idea to check it out.
Lane [ l a n e @ p a i r . c o m ] "Straights are for fast cars. Turns are for fast drivers."