The first 4 plug connector with the heavy gauge red wire is identified as C497, I'm using 87 Cutlass manual, but it should be very similar or even exactly the same as MC, GP, Regal HVAC wiring with RPO C60 (Four Season).
C497 terminal B comes from Fusible Link A (starter solenoid presumably) and goes to the A/C Blower Relay terminal D
C497 terminal D is the black wire that comes from the Compressor Clutch connector, and provides ground for the clutch coil.
C497 terminal C is the light green wire comes from the dash HVAC control head, and goes to the compressor control relay to provide it power. The ECM controls the compressor by ground that CC relay.
I don't remember but I don't think there's any fourth wire in that C497 connector so it's just an empty cavity. Maybe someone else can chime in if they know.
Lastly looking at your compressor clutch connector, that looks like its for a later year R4 compressor that has the metri-pak plug, vs the older one with diode taped inside the tape in the 80's cars. If that's what you have I'm sure you can use it, just be sure it has a diode in the harness, or you can easily change it if you need the older style.
Hopefully this information is helpful. I know there were a few variations on compressor/blower controls on 80's G Bodys depending on carline/engine, etc...but by and large they were mostly similar in control logic. The CCC cars (with ECM controlled carburetors) typically controlled the compressor with a relay and A/C inputs. Compressor safety/cycling (high and low pressure) were typically controlled via hi and low side switches. Early GM used to use thermostatically controlled cycling switches with capillary tube sensors on the evaporator outlet, but I think those were all mostly gone by mid 80's...