Heater Box-A/C question

I'm converting my '93 chevy no option work truck into a Silverado. Swapped in a '05 LS 4.8L and added cruise. Now I'm adding air. I bought a '89 Silverado for a donor truck for the heater box and the controls. I'm using a Sanden A/C pump. My question is how do I get the recirculating door to close. I'm assuming it should be closed during A/C. From the wiring diagram I have it looks like it activates from the air inlet pressure switch that's in the back of the original R4 pump through the A/C heater contoller to the inlet air valve motor. If I'm correct and I'm not sure that I am, using the Sanden A/C pump, do I need to install a pressure switch somewhere in the system and wire it into the A/C heater controller or am I way off ? Thanks for any help.

switch in the back of R4 compressor is a high side cut off safety switch to turn compressor off. Back than I think wiring went from control head to the recirculation door, if you touch just A/C air was from outside. If you touch max A/C control head would shut door for inside air.

Thanks for the info. The '89 control head doesn't have a max A/C button, but I found the owners manual in the '89 donor truck. It says the system will automatically determine if it needs recirculated or outside air, but doesn't tell you how it does it. Should I have a high side cut off safety switch somewhere in the system? As I'm putting this together all I have is the pressure cycling switch on the accumulator. On my wiring diagram the switch in the back of the R4 pump is called an "air inlet valve pressure switch" and is wired to the "G" terminal on the control head which is labeled "valve close enable". That's what gave me the impression that this switch somehow controlled the recirculating door. Let me know what you think. Thanks again.