Made my Dad some custom tail lights for his old car, it's a show car all modified, 600HP, etc.
So we worked on a project to retrofit some LED lights.
It uses an arduino but not any traditional board, I designed my own board with the LEDs and Maxim chips (Used for LED control) all built in, also added a USB port for programing and since there are 2 lights and you want them to be in sync, there is an I2C wire bus running between the 2.
Since the car will be at car shows I added a fourth function of a remote control to switch on a 'Show' mode that cycles through some displays, again using the I2C bus to keep the 2 in sync since resonators aren't perfect and there is a trimmer to set the speed, it's almost impossible to get them set the same speed, so the drivers side tells the passenger side light where it's at and what it's doing at what speed.
Made a nice all shrink wrapped wiring harness with molex connectors so it would install just like the stock lights, here is a video that show one in operation, they have 4 modes: Brake, Turn Signal, Tail light, and Show Mode.
The black box in the video uses tiny switching power supplies (Less heat, practically none) to convert the 12VDC signals to 5VDC for the board, also contains the remote hardware.