You can also look up "Balloon Lights" - I've seen those as well.
I did a Dr. Who Tardis cake, with a flashing blue light on top. It was a home-made wired job with a small battery pack. I put a straw through the cake to the back of the bottom and fed the wires through that and hooked up the battery pack to the wires then covered it with a little fondant. There was a switch there, and it worked great!
The aqua lights would probably work fine for the car, you's have to set them up to be able to turn them on and slip them into the hole in the cake covering though. A foil "socket might work to keep the hole in the correct shape for easy applying.