What you need is the Powerdecal. This 4" sticker can be affixed on the inside of any window and uses three AA batteries to provide a constant illumination whenever your vehicle is in motion.
Using a tough 3M adhesive surface you can stick the Powerdecal on the inside of your car window, turn it on, and the sticker does the rest. It uses a motion sensor to detect when the car is moving and glows the image of your choice to the outside world.
Battery life is up to 45 hours, and you can buy the device on Powerdecal's own website. The cost of illuminated allegiance? $24.95. Batteries are included...