Quite right. Heat shimmer is a problem when the outside temp is very low. I only keep the floor duct open and the fan on low. Even then, I position the lens well out of the window portal, often into the acute corner next to the windshield and turn off the engine if I feel it will help. Best case scenario is being able to actually exit the car and walk away from it, but that approach is subject to what traffic will allow. What I don't understand is why traffic only seems to materialize when I spot a subject. No traffic before, or after the event. When it gets a little warmer, like 10-degrees, then I'll dispense with cabin heat.