This is by far the best product I have ever used.
I had an 8 year old Avalanche and the sails were terrible. I used this and it made them like new again. I also use it for black running boards, mirrors etc.
It is fairly expensive, but I am still using the original can i bought 4 years ago.
http://www.collisionrepairsupply.com/rere.html
ps. you don't have to buy from this outfit, it was just the first one that I saw after a google search.