Bama Rambler is 100% correct. The only way to fix a leaking window is to pull the window and replace the butyl rubber tape behind it.
I had a leaking front window and found a corner where the buytl tape was out of place from the factory. Sealing it from the outside would last for about 3 months and then it would leak again. Once I pulled the window and did it right, the leak was gone.
2007 Rockwood 8315SS
2014 Silverado 2500HD 6.0L