SOS Lights

Roof-mounted, up-facing light arrangements (probably ultra-bright xenon bulbs for fog penetration) with which you signal for aerial help.You enter a message into a dash-mounted keypad, like "HELP" "LOST" "HERE I AM", etc... and a small, stupidly simple computer translates your message into Morse Code and flashes the lights in that sequence.

So here's a scenario in which this comes in handy:

You're on a long roadtrip (poorly planned) in the middle of winter, and your route takes you through the Rocky Mountains in Colorado.You find yourself in a heavy fog or snowfall, and miss the sign for your exit. You come to a sign with the same route number as the one you were looking for, but because of the reduced visibility you fail to notice that the sign is for State Road 90, instead of Interstate 90.

You get about 40 miles down the road before you realize you must've made a wrong turn. You turn back around, but on the way back the road splits in a Y, with no signs. You take the left fork when you should've gone right, and by the time you realize you're lost, you have no hope of getting back to the main road, especially in this weather.

You look on your map and notice there's a major airport not far from the last point you remember passing, which means lots of air traffic this time of year. Fortunately, you keep an emergency flare gun in your glove box. You stop where you are, get out and shoot off a flare (and somehow manage to avoid shooting your dumbass self) through a special opening in the roof. Then you punch an SOS message into the keypad on your dash, and the signal lights start flashing your message. The pilot of a passing airliner sees your flare and decides to make a pass over the area. He sees the bright signal lights on the roof of your vehicle, deciphers the message, and radios the tower at the airport with your location to send out a rescue party.