Ok, a while back I listed some ideas that, I thought were cool. http://jeramysonelife.blogspot.com/2006_01_01_archive.html
One in particular:
Next Idea; just about everyone drives, and everyone has something to say. How about a lit scrolling display that fits at the base of your car’s rear window. It is programmable so you could put in any message through a remote. And at a push of a button you can tell the driver tailgating you to ….FUCK OFF ASHOLE…. In bright scrolling letters. You can advertise ….Buy Tamales at MADRIDS GORMET TAMALES… Or get a date…HEY HOT CHICK, CALL ME…..206-555-1212…. Just imagine how fast this item would sell. With today’s technology this item would be as cheap as a calculator or some Chinese made Walmart thingy. Hop on it folks! Just don’t tell me how much $$$ you have made. http://www.uspto.gov/
Well someone had the same idea and is making money off it right now. Yea!

Well here is the link if you want to buy one.
http://www.au-my.com/product.aspx?lang=English&product_id=1