It happened again Tuesday, so I decided to finally attack this problem.
Four or Five years ago on a heavily raining day, my wife got in my truck at the park-n-ride to borrow it for awhile. She turned on the fan and water came shooting out the defrost vents. Then she found a very large puddle on the passenger floor mat (costco mats which hold the water). Dealer tested it for the TSB related to water on the passenger floor area and hosed it down real good with no luck of reproducing it.
Years go by and it never happened again...until this Tuesday. So I was wondering if something was done incorrectly when I got the new windshield a couple of months ago (although I doubted it since it happened once before). Still, there were people claiming that the wipers and cowl should be removed when the windshield is replaced. That's a wrong assumption and I'll show you why in the pictures. It's actually better if they don't remove the cowl.
So here's a shot of the cabin air intake box with the cowl removed (you can just seem my DIY washable cabin air filter in the pic):
The Cowl has a channel in it that the glass slides into just above the cabin air intake box. Thus the need to leave the cowl in place when replacing the windshield. This is a perfect guide to ensure they put the glass in the correct position (height-wise) so you don't have trouble re-installing the cowl or leaving too much of a gap for water intrusion purposes.
Below is a closeup of that channel on the cowl that slips under the glass. Notice it has tabs that slide under the glass. This way the glass sits against the top of the cowl leaving a channel below it for water evacuation away from the cabin air intake. The water then drains off the ends. However, this is where there are a couple of faults in the design. I'll get into that after the picture.
Okay, so there's two problems with this design. One is that at the end of the channel, water could run down and back underneath onto another lip below the channel back toward the cabin air intake box. There's a vertical fin that's part of the cowl mold that would stop the water from continuing on, but that fin still sits about 2 inches too far toward the intake box. That could cause water to drip into the box.
The other problem is what I believe some of us have suffered from. Depending on the slope you're parked on, this channel may not be big enough and it doesn't have a back lip (windshield side) to stop the water from pouring over the edge anywhere along it during a heavy downpour (or if you get debris in the channel, you're screwed). Notice how open the channel is in the pictures. I think this is the problem I've suffered from twice now. Both days there were major downpours from thunderstorms. My guess is that it just can't handle it.
My solution? Install a piece of flashing directly over the cabin air intake that will always direct the water away from it. I shouldn't have any more leaks now.