I think maybe its your selector motor. (couldent be really positive) having the same problem i can change my vent to blow cold or hot air or what speed i want it to come out but cant change off of defroster. There is a control motor back in the dash it would require taking the dash out but i think if you change that you'll be free to select the output.
Have you changed this motor on yours? What yr do you have? THanks