No guarantees here, but if you are getting virtually all the air flow out the defroster duct regardless of the setting of the controls on the dash, it might be a failure of the dash controls. The dash dials don't control the heater directly. They tell the computer what you would like to happen and the computer makes it happen. If the computer has doubts about what it's getting from the dash controls, it reverts to the defrost mode by default, and not necessarily full hot temperature.
Replacing the dash controls is a relatively easy job. Pull directly rearward on the bezel that surrounds the control panel and the radio. Use your fingers. That will pop the bottom fasteners (two). Then using a plastic trim tool (best option) or a putty knife, pry the remaining clips loose. The bezel comes off carrying the controls for the heater and leaving the radio behind. You then unclip the wire harnesses and it's out.
Replacement parts come up on eBay fairly often, but if you can't wait, try a Chrysler or Dodge dealership. The Dodge Stratus uses the same part.

I would try replacing the control head first, because if that works, great. If you have to replace or repair the heater system, it involves tearing the whole dash out and it's big bux.

 

Thanks again, great and informative reply. I thought the dash control were directly linked but now it makes sense. Sounds sensible to do in that order and even with my level of mechanical know how. I'll start to look right now and hopefully that'll fix it as I don't think it's going to be worth be trying to repair the heater system as I'm thinking of trading in after the winter anyway.