1. A method for automatically controlling a heating, ventilation and air conditioning (HVAC) system of a vehicle which discharges a flow of air to a passenger cabin of the vehicle to prevent fogging of a window pane of the cabin, the system including a variable speed blower, means for varying air temperature, ducting, actuators having various control positions for controlling the direction of air flow and the ratio of fresh air to recirculated air, a humidity sensor for sensing relative humidity within the cabin and temperature sensors for sensing temperature within the cabin and ambient temperature and an adjustable set point temperature, the method comprising the steps of: determining a fog condition boundary based on temperature within the cabin and ambient temperature; determining a difference between relative humidity and the fog condition boundary; determining a fog probability based on the difference; defining membership functions and fuzzy rules between the fog probability and the speed of the blower and between the fog probability and the control positions of a first set of the actuators; and generating control signals to control the positions of the first set of actuators and the speed of the blower to cause the system to discharge air into the cabin having a desired temperature and flow based on the fog probability, the membership functions and the fuzzy rules to prevent fogging of the window pane.