1996 Ford E-Series Van Climate control panel

The vent/floor/def/ac selector slide does not activate the correct position. It is stuck in the defrost position and will not change. The temperature selector works fine in heat/AC cold to hot ranges. I assume it is an electrical/vacuum activated selector. Where do I check for vacuum leaks or faulty solenoid activator? I have the panel out and the color coded vacuum lines are in tact to the selector lever bundle but go deep in to the dash to where? There is also a vacuum driven diaphram on the right side of the blower motor which is accessible under the dash on the passenger side. Is this part of the same system? How do I check for vacuum leaks and trace for any?