double-check the fuses... most commonly a blown fuse is the culprit
Join the fucked up car club!
Diablo and I are already members, and we are accepting more members daily!
Bottom line is, it's not a fuse. If it were, then the radio etc, would not work at all. Diablo thought it was a relay due to the fact that the only times the lights wouldn't work was when the headlights came on at night.
He got a new tail relay, and still nothing.
No one knows what causes it or how to fix it. At least no one online.
Probably have to take it into the dealership to get it fixed, but that may not be worth it considering how much it may cost.