Previous car had a Cognac MB Tex interior. Cognac is a light color that shows the dirt.
The seats had not been cleaned for many years. The usual gentle cleaning methods did not work. I used a fairly stiff bristle brush, a lot of elbow grease, and Lysol full strength. Lysol is used to clean public restrooms so you might not like the "fragrance" but it worked very well. I did not try Simple Green but my guess is that it will work as well and not insult your nose as much.
Richard M, NYC