Are you able to change blower speeds? If it blows on one or two speeds and not on others, chances are it's a resistor block....
If the the A/C only comes on while in the defrost mode, it tells me there is nothing wrong with the compressor, or it's controls(pressure sensor, relay, etc.).....
Does the mode switch work? Does air come out of the vents when commanded to the vent position?
As for the temp not changing, that could be the electrical temp actuator or the control head......
Only a high end scan tool capable of reading BCM data, and trouble codes, can tell you if you are getting the proper commands from the control head.....
Only proper diagnosis, can save you from replacing unnecessary parts....