I have a 99 f150 xlt and when I select any functions to the right of the off position (vent, defrost) the air condition electric clutch toggles on and off. Is there any reason why I can not put an external switch to de-activate it, then switch back on when I wany to select a/c. ?

 

It should not turn on with the vent. You could do what you are purposing however the defrost works better with the compressor running. I would leave it. KISS.
Jes

 

The AC clutch should be disabled in Vent and Heat modes. all others, the compressor runs.

 

There is no "mechanical" reason why you cannot put a separate switch on.