I've never had any luck with upholstery cleaner from a can. It looks great after application until a drop of water hits the seat. Then all the dirt comes back to the surface.
Most hardware stores will rent steam cleaners for about $20 a day or so, and that's what I do when I need to clean the interior. They come with an attachment for upholstery and work very well. However you will spend a bit of money on the cleaning fluid they need so figure $60 or so for the whole job.