There are a few things that will automatically engage AC or heat and they will even kick in unexpectedly at other times for no explainable reason. Hitting the Max defog button will engage AC without lighting up the amber light. Try specifically selecting one of the airflow settings (defog, foot, or mid-dash) and then press the Heat & AC button to turn it on and then turn it off. Then hopefully the next time you turn the car on the same settings will be in play.