How can I add such a switch? Shouldn't be too hard right? I mean, as far as I can tell, all the air flow settings also turn on the compressor.

 

heater settings should not start compressor

 

For 95~11 Rangers w/AC, compressor operation is allowed or disallowed based on the mode selector switch position, not on heat setting:

AC compressor operation allowed:
Max AC
AC
Vent/Floor
Floor/Defrost
Defrost

AC compressor operation not allowed:
Vent only
Floor only
Off

If desired, it is possible to add a dash switch to disable compressor operation. The wiring change depends on the year of the Ranger (95~11).





[94-back Rangers w/AC came from the factory with an AC switch.]

 

Here are some instructions I gathered from another kind member. Sorry I can't remember who to give credit to. This instruction will give you an idea how to do it one particular way, and also depends on your switch selection.
FordRangerACswitchMod.docx