Don't know about that drain either. But there are drain holes at the bottom of the door. If the water fills up in the door and can't run out, I guess it could end up coming through the door panel into the car. But if the drain is working as it should, the water should be able to get out into the door jam and go outside. If the water is actually coming into the car, my guess is the window. Looks for clues around the car and door for water... you may see something.