Update: My 2015 2500HD Duramax LTZ Crew Short (last 8 of the VIN: FF558385) was in the shop for a week. They finally isolated the issue to water penetration at the top of the passenger side windshield pillar. They removed the windshield and repaired/resealed the seam at the top corner, put the windshield back in. They retested and stated that it was the problem and is now dry. We will find out as I am headed from Denver to Chicago on Sunday and there is going to be plenty of rain. Not sure how it left the factory leaking so bad but maybe they really don't test them.