i know this is a bit off topic but, i know of someone, who knew someone who was at the pub of his face when he got into his car he threw his guts up all over the dash and the vomit went down into the heaters ect... didnt matter how much he cleaned it up the smeel never went away, even after pulling the dash apart, never killed the smell, ended up selling the car. Obviously they didnt have the heaters or aircon switched on when they test drove it... very unlikely this is you case but could just well be the case... hehe
you might wanna refill your aircon, a mate had a falcon and it smelled abit when it was empty...
by the way its winter, what you need aircon for?