I have to deodorize/disinfect my AC about once a year, mold tends to grow in the vents because the cool air causes condensation in the system; and where there's moisture, mold will grow.

I've use a product called Clean Air from a local auto parts store to fix this problem. You have to spray it into the air intake and the vents.

A way to avoid this problem is to leave the blower on with the AC off to blow off the cold air before shutting down the engine. I'm just way to lazy to do this every time.