There is a product on the market called ODO-Ban. Spray the area with the mold or mildew with a full concentration of this. It has a pleasant fragrance, but may be a little overpowering for a while. However, it will cure your problem. We had a pick-up truck that developed a leak around the windshield and mold formed on the carpet. This took care of it. Good luck!
I'd try putting some charcoal briquettes in a plastic bag (leaving the top of the bag open) under the seat. After several days, I'd replace the briquettes with new ones if the odor wasn't gone. Charcoal absorbs ordors.
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