Ford Repair: A/C vaccum/defrost to vent, vacuum gauge, engine vacuum

Question
1993 ford mustang lx 4 cylinder automatic trans.
The problem I am having is that when you switch from defrost to vent, it doesn't switch!  I have checked all vac lines and replaced the bad ones.  It seems like I am not getting vac to the switch on the dash.  I can reach under the drivers side dash and manually make it switch to vent by messing with the little canister.  All the vac lines under the dash are hooked to something.  On alldata it talks about s0metype of electronic vac switch, but of course it gives no location, and I can't reference the part number on autozone.  I need help I have the same problem on my 1990 f350, my A/C instructor doesn't have a clue of the problem.  I am lost too, it's not like my 1972 ford which is so simple.  I need the air to come out the vent instead of the windsheild and floor on the fords.

Answer
Hello and thanks for the question...I'll try to help.
 What I would do is start at the engine vacuum port to the interior and work back. Use a vacuum gauge and a hand pump....test the connections and lines......and of course the vacuum signal at the engine port.
Erik