This problem is with all pickup truck-based GMC vehicles in the 1500-3500 series from 2010 to present. Margaret in our Ask a Mechanic forums brought this issue up with her 2011 GMC.
Big Block 409 quickly answered by posting a service bulletin (TSB 06-01-38-003), which deals with several air conditioning and defrost problems, including this one.
Pinboard (MacAaron)
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