Well, yes, it does overflow "up there", but being water, will follow the path of least resistance until it can't go any further. In my case, it ran down the inside of the A-pillar. From there, it can flow to other places, depending on how the car is moving, or parked (ie on a upward hill, downward hill, one side higher etc). Water can also wick through fabric, and cause things to be wet far from the original source.

 

I have to blow mine out every November before the California rainy season. As long as I do that they are ok for the season. Just pop the cowls off on both the driver and passenger side, open the sunroof a crack, and blow compressed air into it. I also got one of those battery powered air guns and that works as well. I find the rears dont block as much.