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.
If it detects no movement for 2 minutes, it switches off again. It'll also only illuminate when it's dark, since in bright sunshine it'd naturally lose its impact - though of course it'll still act as a regular sticker.
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...
[Gizmag]