Robert Dunn soldered 480 red LEDs into a grid in the rear window of his truck:
The LED Sign itself is a 48×10 soldered together matrix of red LED’s. I soldered them together sparsely in a mesh so it could be mounted in the back window without losing rear visibility. The rows are controlled using a 4017 decade counter for POV. The columns are controlled using six 959 shift registers.
Input from the vehicle is recieved from the trailer light hookup through a cable and into the cab. The signal from the trailer lights hookup is connected to the arduino using optocouplers. The arduino can sense both turn signals and the lights using this setup. The 12v signal is regulated down to 5v for the optocoupler using 7805 voltage regulators. There was probably somewhere in the cab to get the turning signal input from, but it was easier to use the trailer hookup, although uglier.