Mold is virtually indestructible. The only thing that truly kills it is bleach, and even then, if it doesn't all die, it just grows back. I don't know if you can clean your convertible top with bleach, but you need to weatherproof it after you are done. Keeping it dry and keeping it in direct sunlight can help to kill the mold as well, but I really don't know if that will work. Try lemon juice too.
