You have tried air freshener after air freshener, but your car still has the wet smell of mould. You tried to air the car out, but that only helped for a while. In order to permanently remove the smell of mould from your car's interior, you must first identify the source of the mould and then eradicate that source. Mould is a living thing, and simply cleaning the traces of mould from the seats will not effectively address the problem. You must eliminate the source of the mould.
Begin with a clean car. Remove floor mats and any belongings. Vacuum the entire car so there is no loose dirt. Mix the cleaning solution 473 ml (2 cup) of vinegar to 59 ml (1/4 cup) of hot water -- in a bucket or deep pan. Thoroughly wet a rag in this solution. Vinegar has powerful cleaning properties and will effectively kill the mould spores if used properly. Ring the rag out so it is not dripping, and scrub the entire car. Make sure you don't neglect areas that are hard to reach, like under the seats. Do not hesitate to scrub fabric seats, as there may be mould spores living comfortably within the fabric. Use a soft bristle brush to work the vinegar into the fabric so it can effectively reach the infected areas. Remember, your goal is to kill the mould so it can no longer reproduce. Allow the vinegar solution to remain on surfaces for about a half hour to ensure contact with possible sources of mould production.
Now that you've successfully eradicated the source of one smell, the mould, you have another to contend with: vinegar. Vinegar is an excellent, natural and environmentally friendly cleaning product, but it is not exactly a desirable smell for your car interior. Luckily, baking soda is an odour neutraliser. It is also natural and environmentally friendly. Mix 3 tsp of baking soda with 237 ml (1 cup) of warm water in a small container. Wet a new clean rag and scrub the car surfaces. Make sure the baking soda solution makes contact with everywhere the vinegar solution made contact.
Do not allow the baking soda solution to sit on the interior surfaces. Fill a water bottle with cold water and spray down the surfaces where the baking soda solution has been used. Do not soak the seats, but get them wet enough to effectively rinse off the baking soda.
Hand-dry the seats to the best of your ability. It is important that the moisture in the car does not linger, since moist environments are ideal for mould production. If it is a dry, sunny day, consider letting the car air-dry, or use a hair dryer on a powerful setting to speed up the process.
Vacuum the surfaces again after the interior has dried completely. Since baking soda becomes a powder when dry, if there is any baking soda left on the surfaces of the car, vacuuming will pick it up.
Your car is clean and the mould has been eliminated. Now you may want to try a spray air freshener to eliminate any traces of vinegar. Use moderately.