First - change the air filter - the one for the ventilation system not the engine - second go to your dealer and purchase an aerosol can of air con cleaner - usually �10 - �20 and use as per instructions -
generally you put the air con on max and recirculate and place the aerosol inside the car and press the button on the aerosol which empties the entire contents over a short period of time and gets into the air con sytem cleaning it - so i've been told - never had the need to do it yet.