I left the sunroof open on my W203 and it rained
Here's the damage:
http://i.min.us/icJ6QA.JPG
Anyway, I would look into buying replacement seats from a salvage yard. Probably same expense and less work than reupholstering.
Use a 10:1 (or stronger) solution of P21S® Total Auto Wash and distilled hot water, let solution react for 5-10 minutes to enable cleaners to react, you may have to repeat this process



