like ozzy said u take out the cabin filter and spray it into those ducts..then set your ac to recirculate the air in the cabin. i use FRIGI FRESH.. its made for that. or you can buy those 1z kits from KLIMA-clean where u spray it down your ac vents. you need start getting onto the habit of shutting off your ac a few minutes before u get to your destination otherwise the smell will come back even after you kill it with products.
http://www.1z-usa.com/einszett_klima_cleaner.html