It is really simple. There are two drain hole beneath the plastic slide, one on each side of the sliding glass. Pull up the plastic slide to expose the drain hole (there is one in the slide as well, but it is not over the drain hole in the seal). Take a small wire tie and push through the hole. If it is all the way through, you can see it from the out side. I then sprayed water on my rear window and I could see the water drain out side of my truck under the window on both sides.