Car seat cleaning
Cleaning the interior of a car includes the seat cushions. Here we have to differentiate between fabric and leather. Soiled fabric covers can usually be cleaned with an upholstery foam cleaner. But be sure to follow the directions and test it first on an inconspicuous spot to ensure that it doesn't ruin the color. If it discolors the fabric, it's not suitable to use.
When cleaning a stain, it's always important to treat a large area at a time - for example, the whole seat surface. Dirt is transported by liquid during cleaning; the result is an unsightly edge left after it dries, a so-called "water spot". Another way to prevent this is to soak the stain and rub it with a dry cloth from the outside in.
For heavy soil and deep-seated stains, you should leave the cleaning to a professional so that you don't damage the seats. Nowadays car seats can contain many electronic components which must be protected from liquid or other damage.
Special care for leather seats
Cleaning of partial or full leather seats takes more effort than with fabric seats. First the surface has to be wiped off so as not to seal in the dirt later. You can buy special care products in the automotive accessories market. Never use saddle soap to care for a leather seat, as it will cause the leather to become weak and begin to warp. Just as with fabric seat covers, test the cleaner first on an inconspicuous spot to ensure that it doesn't damage the leather. Follow the directions and use cotton or terry cloths. Many care products replenish oils in the leather, and with these no further steps are necessary after cleaning - this is of course only for smooth/buffed leather, not suede.
If the product does not have an oil-replenishing effect, special treatment for your leather seats comes next after cleaning and drying. A care product with a wax or resin base is particularly useful; apply the product sparingly with a sponge. Next you'll have to let the seats dry once more before finally polishing with a soft cotton cloth.