1997 Ford E350 (Triton V10 6.8L) A/C problem

Whenver i try to change my ac to the dash or the floor all it does is stay blowing out of the defrost vents on the dash. I dont have a vacuum leak but i think that the vacuum actuated part that controls the vents on the dash is bad. What is this part called and where is it located? Please give lots of detail in describing the location of this part.

Thanks for any help