You could use those expensive products, but I find for most shampooing, landry detergent works great diluted in warm water, using a scrub brush and vauum cleaner. I like ERA the best and a jug will clean a fleet of cars. One cap full to a gallon of water and change the water frequently. For really tough stains a degreaser is inorder, like Castrol Super Clean, Mr Clean, or any other house hold cleaner. Once scrubbed and vacuumed, give them a good rinsing with the garden hose or in the sink, to remove all the dirt and soap. Using specialty auto cleaners is just more expensive, not better. I used work at a car dealership and cleaned cars for a living, used all the industrial products, and still prefer this method. (plus it is kind on the lungs and hands)
Greg