Defroster problems 1972 Chevrolet PU

[w6tractor]

I am confused/frustrated wth my A/C-Heater. Everything works perfectly, except the de icer. When I move the controls to the position to de ice, the A/C compressor does not come on. I have had all the A/C/Heat ducting etc. off and replaced. The cables go to the correct places. The switch on the underdash heater box that controls the compressor works, but not when set for de ice. Everything else switches from vents to floor to windshield outlets. How do I get the compressor to come on in the de ice position? What have I missed? Thanks