I was refering to the dash knob used to control the cool/heat temp that comes out of the vents!
The dash control knob may not be opening the blend door to allow heat to the vents. What happens if it is all the way to the hot side?
I had an F150 that had a simular fault. The fix was to replace the seal (under warranty) for this blend door. That was a 2005 though, so I do not know if our current mustangs have this issue. I'm off to check the repair manual to see.
It is better to keep your mouth closed and let people think you are a fool than to open it and remove all doubt.