My heater quit working on my 2006 Dodge 2500 5.9L, as well as air only blows out of the dash vents. Regardless of where i turn the control **** to it stays in the vents. No air will come out of the floor or defrost vents. When i turn the control **** to defrost the air coming out of the vents gets colder. A/c is off and feels like i have it on when turned to defrost position.

 

The doors in the ducts fail quite often in our trucks. I would do a search on HVAC or Recirculation door issues. Not an easy repair. The Heater-Treater replacement doors are an option. They also discuss the issue on their website. Just google Heater-Treater.

Hope you can get it fixed before winter.

 

Sounds like broken blend doors, and probably some others.

Removing the dash and HVAC case is about 5 hours work, and 5 to put back in.

There is a thread titled Hvac removed and understood which is a good write up about what is involved.