Same problem here, not sure if same cause
Post Heater does not work when cold
Still not fixed, my temp never got to 80, I replaced the thermostat. Now it does but still no go on the heat. I am told so far it is the "DUO" but I don't want to spend the $436 until I am sure it is it and not any other parts.
I would think if your front defrost works but blows cold air all the time it may very well be a thermostat. Since I was told once that the front defrost should work in any case.