Check that the battery is not over charging. When it is over charging it vents right next to the heater intake. Mine did it due to a faulty a regulator. I was on a road trip when it happened. The Canadian tire guy said I would be O.K. until I got home. WRONG. It ended up causing a power surge and frying the stereo, power antenna and every light bulb in the car. Get the problem checked out ASAP.