Here is a test turn your truck on and turn the air on PANEL HIGH FAN WARM air and have all the windows up and all the doors closed. Then get a spray bottle a fill it with real bubbly soapy water. Then spray all around the seals exterior on the back window on top of the window and around the cargo light if applicable. The soap will bubble when you hit your leak. If the window is leaking you can reseal it by removing it First remove the seat back and the trim around the c-pillars and the interior overhaed light then remove the nuts for the glass get some help.