Probably your local free ranging tom cat. Just kidding.
Changing the cabin filter often helps, but often you should do more as well. You need to kill the mold and bacteria that can grow on the evap core and in the a/c plenium. Dealerships have a special disinfectant and a special sprayer to apply it. The sprayer has a long hose with an ultra fine spray nozzle. Usually the resister pack is removed for access. After application, the disinfectant should be left without using the fan for at least 1/2 hour.
You can often do a lesser but usually acceptable job with Lysol spray while you have the cabin filter out as you are changing it. Choose a Lyson scent that you can live with as it will be strong for a few days. With the cabin filter out, turn the a/c fan on at a medium speed and spray the Lysol into the chamber thru the cabin filter port. Use about 1/3 to 1/2 of the can. Shut everything down and finish installing the cabin filter. Allow to soak in for at least 1/2 hour. Even longer is better, such as overnight. Remember that you want to kill the micro organisms that are generating the odor. If you don't like the smell of Lysol, then the cure might be worse than the problem.
Wynns sells a product called Aeroscept (sp?) that works very well, but it is intended for professional application and would be nearly impossible to find for a do it yourselfer. Pep Boys used to sell a product called the "Odor Gun" that was scentless and worked very well, but I haven't been able to find it for a couple of years.
It does help to turn off the a/c and turn the system to vent about two to five minutes before shutting the car down. That helps to dry out the chamber so that the moist conditions that the micro organisms thrive in.
In extreme cases, the evaporator core must be removed from the car and the outside thoroughly washed with a bleach and hot water solution. I have never seen a Contique that needed this extreme treatment though. On the cars that did need it the growth on the core looked like a fine green moss.
Also for extreme cases, there is an "afterblow" kit available. The afterblow kit turns on the fan for about 5 to 10 minutes after the ac has been shut down and the car is parked for about 1/2 hour. There is a TSB about it. If you are out of warranty, it is a bit pricy, but worth knowing about.
