Does the A/C come on when you have the control module set to A/C? If the A/C compressor kicks in and works in "normal A/C" then it could be the control switch. The switch sends a "request" for A/C signal to the Computer along the same wire. If the A/C compressor comes on in "normal" mode and not in "defrost" then it could be the control switch.
JMHO
