It might not actually be a stain at all but it's very possible that it's clean and looks like a stain since the rest of the seat is dirty. Here's some things you can try.
Removing common stains
To remove lipstick stains from your car's upholstery, try rubbing gently with a white, non-gel toothpaste. Then wipe clean with a damp cloth.
If there is a battery acid stain, rub a paste of baking soda and water into the spot right away. Leave this on for two hours and then wipe it off with a damp cloth. Repeat if any stain remains. Any remaining residue can be cleaned with a commercial upholstery cleaner. As an alternative, you can make your own cleaner by mixing 1/2 teaspoon liquid dishwashing detergent in a quart of warm water. Beat this with a mixer and clean the upholstery using only the suds. Work on a small area at a time, overlapping areas to avoid spotting. Change the rinse water frequently to keep it clean. Let dry thoroughly.
To remove a gasoline stain from your car's interior, treat the stain with a mixture of one teaspoon each of vinegar and mild dish detergent in a quart of warm water. The vinegar will remove the odor while the detergent does the cleaning. Let the area dry. If any spotting remains, you may need to repeat the process. If this does not seem to be working, try using dry cleaning solvent.
If you have children, they may play with crayons while in the car and may get some on the upholstery. To remove crayon marks, first scrape the excess crayon off with a dull-edge knife or metal spoon. Spray with WD-40 and let stand a few minutes. Using a small, stiff bristled brush, work on the crayon stain and then wipe the area with paper towels. Respray with WD-40 and apply liquid dishwashing detergent on the sprayed area. Work this material in with the brush and then wipe the stain away with a damp sponge. If any stain remains, repeat the procedure.
All that eating and drinking in the car can result in stains from liquids. The first step is to blot the spill and absorb all excess liquid. Then apply an upholstery cleaner available from your local auto parts store. Spray a small amount of cleaner on your car upholstery and wait a few minutes. Scrub the stain a little with an old tooth brush. Then dry the soiled area with a clean cloth. Repeat until you no longer see the stain and the cloth you are using to dry the area shows no discoloration. If the stain is small, shaving cream may do the trick instead of commercial cleaner.
If ink stains your car seat, do not rub the stain as this can smear the ink and make the stain larger. Start by carefully blotting the area to remove any excess ink. Spray a small amount of hairspray on the stain and let it sit a few minutes. Take clean dry towels and wipe the area. Repeat as needed but use the minimum amount of cleaner necessary. Change your wiping cloths frequently to prevent soiling the material from cross contamination. Rubbing alcohol can also be used to clean ink on car upholstery. Dip a cotton swab into the alcohol and apply it only on the actual ink stained area. Then wipe with a clean cloth as above.
To remove mold, mildew and their odor from upholstered auto seats, products with peroxide and detergents will restore the car interior. This both removes the stain and deactivates the odor. Simply spray a citrus cleaner product on the soiled area. Wait about five minutes for it to penetrate. Using a clean white absorbent cloth, blot the area, pressing down firmly without rubbing for 30 seconds. Repeat this blotting process until the area is dry. If the stain or odor persists, repeat the process. As an alternative, you could create a cleaning mixture by combining 1/4 teaspoon of color safe bleach and 1/4 cup of 3 percent hydrogen peroxide. With a clean cloth, gently rub the stain until it is gone. Rinse the area with clear, warm water and dry thoroughly.