This is a common problem that is most often caused by the salt in perspiration. It can, however, be caused by water that has high chemical content (very hard water). To clean the cloth start by vacuuming it. Then take an upholstery brush (or any moderately stiff brush, and while vacuuming the second time, brush the white areas vigorously. This should remove most of the stain. Next, use a quality car upholstery cleaner – there are many excellent brands available at your auto parts store. Follow the directions – making sure that the interior of the car is not overly hot or cold. Also make sure that you will not need to drive the car until the seats are completely dry. If you sit on the seats while the cleaner is still moist, the cleaner may actually remove the dirt form your clothes and bed it into your freshly cleaned seats. NOTE: Consider a second treatment with some spray on Scotch Guard. It’s cheap and easy, and really makes a difference.