Should always put the aircon when demisting and really at all temperatures. Even if its winter and you have the heaters on the AC/Climate should be on. It drys the air when it passes over the evaporator. At the back of the evaporator will be a little rubber hose that comes out from the bulkhead in the engine bay that kets the oisture that is drawn from the air out. If this is blocked with muck it can back up the pipe and cause damp in the car.