Bleach and water....
I use bleach and water diluted about 1:8 or 10, depending on how bad it is. You just wipe the seats down with the bleach and water mixture and let them sit. Come back the next morning and take a look. The bleach kills the mold and milidew, then the seats show their original color again.
You can also go to ebay and look for Great Lakes Skipper. He has a store on ebay with some great deals. I saw he had the yellow seat backs, not the bottoms. It's been a while since I visited his store. If you have a hard time finding him, give me a shout and I'll try and look him back up for you!:cheers: