Defrost

[rayfinseats]

I rewired my 83 k10 w/ac and want to know what makes the defrost turn on since I eliminated my selector switch. I removed the AC and made my own metal dash. I have a cable to turn the heat on and off but I want to know what sends the head up the defrost vents. Is there a flapper door or something I can use inside the dash? I just want to be able to switch from heat to defrost, nothing else. I get heat down below when I turn on the blower. I still have the defrost ducting but not the center dash ducts. I ditched part 21 and all the parts in the right column pictured.

http://www.lmctruck.com/icatalog/cc/full.aspx?Page=58