Check your doors after a rain, Had a friend in my right rear seat and he heard sloshing, at first he thought maybe just a puddle at the curb lane as I was making a right turn. Nope pulled in parking to get mail and heard water in right rear door, about 2 gallons came out after I ran a zip tie into the drain holes to unplug them. found out the right window seal has a gap at the front door pillar that I will fill with RTV. the other 3 doors are dry. The other 3 seem to be sealed, this one I can run a wire all the way into the door. the rest I cant.

 

I used to have the same problem with my 2009 GMC.

 

Looks like a pretty big gap to me. That should be a dealer fix for sure instead of using RTV.