Better Car Advertising

You joined a network marketing system. So, you need to put a message on your car window saying Do X, Get Y, Call Z. But, you want to be able to switch off the message when you park the car in your company parking lot so that people don't laugh. How?

One way is to use a projector mounted on the ceiling to display stuff on the windows - maybe coat the windows with something that glows when light hits it.

Here's a worthwhile message:

Love your country!
Drive at 55 for better mileage.
Import less foreign oil!