First thing I would do is verify that the leak is not traveling from the source down a support bar to the site you experience the leak. Get in the car and have someone else spray it with a hose. Then I would get this stuff or something like it and spray the top from the inside, make sure to test a hidden corner for color change first.
3M Scotchgard Durable Water Repellent,12-Ounce (5700-6) - Amazon.com