What repels water on convertible car tops?
Hard top or leather? A guy I know used to have a hard top convertable, and turtle wax worked well for him.
Have you seen the ad for 'Never Wet'? Might at least check it out. .
You can use a spray and it repels water, you can buy it at Home Depot
How about a spray on water repellant called Camp Dry. I believe it is used on tents. Read the instructions on the can.