Mold!! I had been living in San Francisco while my precious miata stayed in my parent’s driveway in NY for 3 years at this point. During one of my visits I found mold growing on the passenger seat. A “turbo dryer” intended to keep the insides of boats dry during storage was purchased and left running in my car off-and-on from then forward. I cleaned and disinfected the interior and have never had a problem again. The car’s fondness of letting in moisture from the outside world and it’s storage under a car cover are surely the cause of the initial mold.
Backdate – 08/20/2012