Problem: Small Slit in Convertible Top

Because I'm primarily a furniture upholsterer and don't do much work with automobiles, I normally wouldn't even think about fixing a convertible top. However, a friend of mine brought a convertible car (a 1982 Mercedes convertible owned by one of his friends) that has a rip in the convertible. When he first called up, from his description, (in my ignorance) I had thought that it was something that I might be able to just hand sew up. He said that the owner had recently purchased the car recently (and because of the rip she may have gotten a good deal on the car) and she was just looking for something that wouldn't cost very much. she didn't want to put a lot of money into the repair. She just wants something that will seal it and stop any water from leaking through the rip. The Car is a 1982 Mercedes Convertible