A bunch of super-bright LEDs, some step-down resistors, and some diodes. I honestly don't know which values of anything I used since I built them about 4 or 5 years ago.
If I were going to re-make them now, I'd probably do it a little differently and throw some voltage regulators in them so they're the same brightness if the car is at 11V or 15V, but there's no chance I'm taking them out to modify them.