Found this while bumbling around the internet, comments welcome.
"You can disinfect the a/c system quite easily (and more importantly cheaper) yourself with this trick of the trade courtesy of the Toyota Owners Club forum:
Wiith the air conditioning it will smell if not used every week for at least 10 mins. its a bacteria build up on the condenser.People will sell you (bombs) that supposedly clean it off. these are only a temporary fix like air freshners. I will now let you into a trade secret.
Get an old kettle(you can use a wallpaper stripper, but it takes longer) and put a tray in the front passengers footwell (INTAKE FOR AIRCON) to catch slpahes and drippers fill the kettle with water and rig the switch to keep boiling. TAKE CAR NOT TO BOIL IT DRY, JUST TOP IT UP AS NEEDED. Start the engine and put the aircon on COLD AND RECIRCULATE, and leave boil for at least 1/2 hour.You will see pools of water underneath the car as the condenser is 'washed' . Once 1/2 hour has passed turn off the kettle and put on aircon to normal (outside intake). This will clear any lingering steam/condensation and hey presto one clean smelling aircon system
.It also cleans all upholstery and headlining of dog and cigerette smoke smells. I have done this dozens of times to loads of makes and with a very good success rate. Oh nearly forgot, dont forget to blow out the cabin pollen filter and make sure its dry before replacing it.
(Its behind the glove box on the new shape avensis, and to the left behind the clutch pedal on pre new shape avensis!)
Hope this helps, "