I see that I have been paged!
Anyways I agree with the others that should have had the 3 revision tail lamp gaskets! Mine were put in summer of "14" after 2 1/2 weeks of tearing out everything, overnight mold growth, finally the GM engineers came with some sort of cocktail that finally killed the mold. They did cover a rental for the entire time.
Due to the time it takes to drop the rear bumper and valance, I would let the dealer do it under warranty. It could be mis-formed metal or a missing gasket???
We also have new style 2014 Silverado that the daughter was wondering why the back seat was wet (good catch for a 13 year old) found out that the CHMSL was leaking water into the cab! I ended up pulling the lamp and cleaned the area up and used some household acrylic latex caulk on the gasket area and it has not showed any signs of leaking for the past 100 days.