that cavity has openings in the bottom of it to allow water to get out. I always hit them with the pressure washer at the car wash to make sure they are clear.
It may seem counter-intuitive to shoot water into a spot you don't want rusting, but I think it is better to wash out the dirt that clogs the holes.
After a long week in moab tons of red dirt comes out in one washing.
Looking from the inside (the cavity can be seen from behind the interior panels) there is no rust in mine. I think ti works to keep them clean.