Ok, just compared the WRX manual and the STi manual for the A/C plug. I also looked at the connector on my WRX compressor. According to the manual, two of the wires are used for "thermal protection" of the A/C system, but the STi doesn't have that in the circuit at all. Looking at my WRX compressor, I don't see any evidence of a thermal protection device; in fact the two wires are just jumpered together. So, I'm going to just solder/heatwrap the two wires together, and use the A/C control wire for the compressor.
According to the schematic:
Pin 3 (Blue) is the control wire
Pin 2 (Yellow) is thermal out
Pin 1 (Green/Orange) is thermal in