If you have the HVAC recirculate button off and the air controller set to blow in the footwell you will get outside air ther too, even if the fan is off. The air temp will be either slightly above ambient to hot depending on where the air temp knob is set. This is typical for all cars, not just the Mustang.
2013 Red Candy Metallic GT Premium Coupe
Black Leather, 6MT, Comfort Package, Brembo Package, 3.55 Axle
Borla Touring axle back, Suntec CXP 45% window tint, Eibach Pro-Kit, Steeda Strut Mounts, Rear Upper Control Arm and Panhard Bar, BMR Relocation Brackets, Energy Suspension Shifter Bushing