AC not working properly after new compressor installation

After having a new AC compressor installed by a professional shop, one day the AC button would no longer turn on the AC. The blower switch had been failing as some resistors would not easily select the set blower speeds. Other than that, the control head worked fine. All blend doors, as well as the heater core, had been replaced in 2011. There were no other apparent issues with the controls. The shop diagnosed the control head as the problem. They ordered a 'certified' used unit and then a second one. Neither unit worked. Then, after reassuring me again that the control head may not be the only problem, they ordered a new AC Delco unit, put it in, ran it fifteen minutes until it stopped working, emitting an odor. I was not there so I do not know where the odor came from nor what it was like though the shop said it smelled burned. This had never occurred before. I had checked all fuses, relays.

I trusted them to make a proper diagnosis prior to parts replacement. They made me pay for the 'blown' part to get my truck back. I am not happy with their misdiagnosis.

How should I proceed to diagnose this issue? The blower operates fine. The AC will not turn on and the blend doors will not work. All air flows out the bottom. I appreciate any insights. I am a competent basic mechanic handling fuel and brake lines, fuel pumps and tanks, u-joints, brake systems etc. I can manage a multi-meter and have some basic grasp of this. Help!