Go to a Body Shop Supply Store in your area, they'll have acouple different 3M products that will do the job for you. They can give you a print out on how to use the product.
The real answer is to buy a new one. But you can drill a very small hole in the end of where the crack stops, to stop it from continuing, and then JB Weld the crack from the inside. That will probably hold it. Not necessarily pretty, but it can be sanded and repainted or touched up if you're paint savvy. Good Luck