So basically when the footwell vent on my HVAC is selected it leaks air through the defrost vents as well. l wouldn't have noticed but it was hot and muggy out so l had the AC on at 60. The windshield began to frost on the outside, kinda annoying no big deal. l figure it's a loose hose or a jam. l went to the dealer for regular maintenance and mentioned the incident. Mechanic was baffled, calls Ford and apparently it affects all new mustangs. Service rep tells me its not a big deal and there is currently no fix but Ford is aware of the problem. Wondering how many of us are affected? Next chance you get select the foot only vent and crank the fan to the max setting. Then put your hand over the defrost vents, please report in if you've got the defect