When that happened to me in my last car, it was the multifunction turn signal / lights switch. The connections had worn out and one of the circuits was left open... the rear lights were out completely, but the headlights, turn signals, and center brakelight still still worked.
Try turning on the park lights and wiggling the turn signal around. Do they come on?
then check fuses, your headlight relay, and then follow the harness where it leads to the lights. might be grounded out on a sharp chassis piece. i'd stick to the fuses/relays and the dash area where there might be a disconnected connection or like he said.. a bad switch or the whole multifunctional switch (the lever on the steering wheel)