Help!

My 04' HSE has developed a symptom where only hot air will come from any vent selected.

It feels as is the heat is way higher than any hot air which I recall on the coldest days, it feels like the thermostat has completely failed

This occurs when

- selector under center dash vents set to cold
- thermostat set as low as it will go
- air conditioning engaged - or not engaged

I don't know if it is a blend motor, a control or some other gremlin - just hoping someone can help me trouble shoot

thanks