Either relay stick, switch or harness glitch --a/c compressor comes on when defrosters are requested no matter what temperature you set the controls at --check the controls as well as the blend/mode doors --as to the harness glitch dealer couldnt figure out same problem and disconnected climate control harness for a few minutes and trouble went away {go figure ]

The A/C compressor will cycle if the climate controls are set to send air to the defroster vents, or if the controls are set to an A/C position. Running when on defrost prevents fogging. What is most likely your issue is that the blend door in the heater box is not shifting over completely to the heater core, thus you are getting the colder A/C air rather than the heated air. You could also have a kinked heater hose or partially clogged heater core.