Of the interior of your car the main part of the interior that gets the dirtiest the quickest is the carpet. This is for the obvious reason that people getting in and out of your car will put their feet all over the carpet. As well as this if you have any food or drink in your car and it spills on the floor that can also stain your carpet. There are plenty of ways to keep your carpets stain free and also to clean your carpets if they are already stained. There are many cleaning products now available specifically for the purpose of cleaning your cars carpets. Knowing how to use them properly is important so not to damage your carpets or any of the rest of your interior.
Before using any cleaning products you should ensure any of the substance staining your carpet is removed. So you would need to scrape off any excess with a round edged knife or if the substance is dried used a tough brush. Once the bulk of the substance is removed you can proceed to using the cleaning product to get rid of the actual stain. You would need to ensure all the cloths and sponges you were using to wipe away the stain with were clean and dry for the best results.
Usually when using a cleaning solution you will wet the area first then spray on the cleaning fluid and let it sit for a couple of minutes. Once it has soaked into the stain you can start to rub at the stain with your sponge or dry cloth, you will want to blot it rather then rubbing too much. This is so the stain does not get spread by wiping it over areas that were not stained to begin with.
Although even with cleaning products and stain removers some stains can still remain if this is the case you may need to bring in the help of a professional carpet and upholstery cleaner, there are ones that specialise in car carpet cleaning.