could also be a mouse nesting in there (the relieve themselves where they sleep and eat). it happens a lot when it gets colder outside and/or if your car is outside most of the time...especially at night and for extended periods of time.

spray into the vents, put the a/c on hi fresh air, spray the outside air intake at base of windshield, and don't be afraid to use too much. then put the ac on hi rec and spray @ the inside air intakes (usually floorboard areas). the lysol will kill mold/mildew, and any viruses the mice transmit/carry.

dryer-sheets under the seats and near the inside air intakes will keep the car smelling fresh.