How do I get rid of a musty or mildew smell in my car?

A musty smell usually means moisture is trapped somewhere: wet carpets or floor mats, a blocked sunroof or door drain, a leaking seal, or mold on the air-conditioning evaporator. Find and fix the source first, then dry the interior thoroughly with airflow or a dehumidifier. Clean affected carpets and upholstery with a suitable fabric cleaner and extract the moisture. Replace the cabin air filter, and have the AC system cleaned if the smell appears when the fan runs. Fragrances only mask the problem until the moisture is removed.