I've seen this chronic problem on many trucks. It can be the sliding window, for certain, and cleaning the window drain channels is certainly a good tip. But often it's the window sealant. I have a competent glass man that has done close to 100 windows for me, front and back. I often negotiate group deals, having him come out and do 3-6 at a time for myself and customers, so it's a win-win for us both. The rear windows can be really tricky, it seems. I've had him install new slider windows, as well as remove them because they leak. Even when replacing with a one-piece window, I've had customer's windows leak, which he warranties, of course. But its gotten me to looking to see what I can do to prevent that. No one wants a comeback. Unfortunately, the upright nature of the window, and the fact that you're basically gluing it in the truck, leaves room for the adhesive/sealant to move. Being overly liberal in the adhesive, to the point it has to be cleaned up around the outside after installation, and applying adequate forward pressure all the way across the top of the window until the sealant has dried, has been the only viable technique we've found to work. Many installers don't want to spend extra money on the sealant, thinking they can get it sealed adequately with a smaller bead. No clean up that way, either. Sounds like a good idea, but after the slightest cab twist or even the sealant sagging just a little after install, or perhaps the window flexing outward just a touch, it will leak. I think the biggest problem is that the window is recessed under the cab contour, and it often doesn't get adequate forward pressure to stay against it's sealing lip, causing it to fall backward some, making the sealant separate and leave a gap at the top. I don't know how many I've fixed by sealing all the way around the top of the window, filling the gap between it and the cab. Not the best fix, but certainly cheaper than removing the window and starting over.