Sorry been away on holls for a bit! Even if the aircon is switched off the heating matrix can still cause you problems. This is because on cars with A/C there are 2 matrix's! the first one that the air flows through is the cooling one, this is cooled by the A/C compressor if it is switched on. This system contains Aircon Gas (R134a). If this one is leaking your A/C will not cool and you might get a warning light or fault code in the ECU. This matrix also DRY's the air as it cools it as the water in the air condenses out on the Very cold matrix, then should run out of a small pipe onto the ground under the car somewhere.
This means in wet conditions you can then use the second matrix (the heater one) to reheat the now dry air before it come into the car so you can de mist the car and dry you self out if it is raining! If this matrix has a leak it will put a fine mist of water into the air as it get its heat from the water cooling system of the engine (weather the thermostat is open or closed). This then undoes all of the drying the a/c does to the air and makes it humid, so fogging up your windows.
As I also mentioned earlier it is not controlled by the engine thermostat like the radiator is but is always on so a pig to test. IF you can try squeezing the pipes to the heater matrix to stop the flow of hot water to them this could give you a clue,
as some heating systems let the water through this matrix continue even when cold air is selected as they divert air through an alternate path to the inside of the car, (my old one does this 1992) so it can still affect you even if you turn off the A/C and select cold!
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Last edited by Brendan; 16-04-2012 at 07:28 PM.