The MarkVIIIs use a neon strip in the reflector, the Thunderbirds use LEDs that were custom made by HP for ford.
the neons give a more even glow, but the LEDs would be easier to put into your reflector. In the Thunderbirds they work off of 4 banks of LEDs arranged like [12341234123412341234] each number is a different circut, which is why some that you see out there have stripes of no light.