ok, you already replaced the head unit... I hope they let you return it if it turns out to be somehing simple and the old unit was fine.....
Check if the compressor works by supplying 12V to the compressor switch/clutch. Obviousl the engine has to run when you do this - if the compressor works you can then measure high and low side pressures.... if these are fine then it's purely a electric problem and since you replaced the hed unit it must be the harniss or fuse.... now, is that head unit a known good one or new or from a junkyard ? any chance you bought a dead head unit ???