Someone once valeted my car and used an air conditioning cleaner. It involved placing a canister on the floor of the car, setting the aircon running at full tilt, pressing a button on the canister, and closing the door.
He waited for around ten minutes or so, and opened the car. He told me that the stuff in the canister cleaned the air-conditioning system of bacteria that can produce unpleasant smells. I hadn't noticed any smell before, and there wasn't any afterwards, so I have no idea if the stuff works or not.
I too find most of my spiders take up residence behind the door mirrors.I've tried many times to get them to find somewhere else to live by hosing the mirrors out but within days there's another web built from the door mirror to the window frames.