93 defrost

My 93 Burb will not go to the defrost mode. When selected, it will show the defrost mode then start flashing. The manual I have is not showing me anything. I found the blend door and its working. Havent found the defrost "door". A search of this forum didnt come up with anything. Thanks in advance.

OK, the blend door I foumd was the mixer for the heat. I found the blend door for the defrost, floor and dash vents, it is sticking.