I recently purchased a 1991 325ic for my wife. The top was in horrendous shape and water must have gotten in the car for several years. Once I repaired the top I then proceeded to clean the car thoroughly and found that under the back seat we had dark brown to black wet patches. I sprayed a solution of all purpose cleaner and scrubbed it with a washcloth and left the seat out in the sun for a couple of hours. This took care of the problem.