As I understand it, at this point you have limited knowns: (1) compresor will operate if provied a seperate 12v supply and (2) there is no voltage at the compressor connector when it is believed there should be.well said.
But what about known issues?
This is the perfect situation for the diagnostic expert to determine what is causing the issue with the power not getting to the compressor. They would likely be looking at the in-dash controller, the SAM, the ECM or any/all of the other items that interact with the climate control system. They will also likley look at the wiring/circuit integrity to make sure nothing is keeping the signal from getting through. A problem with integrity can be damaged wiring, corroded connections, or issues with either power and grounds.