I had a similar problem. My 'light out' indication in the dash would consistently display rear brake light out indication. After changing all the bulbs, I thought I had a wiring/part issue.
Then one evening, I go to leave the house at night with my lights on. I back out of my garage, and realized I forgot something. I leave the car running with the lights on, run inside, and when I come back out, I noticed the license plate lights were out. The next day I removed the lenses and the bulbs looked as though they were originals, and the contacts were all corroded. So, I bought some new bulbs, cleaned the contacts, and haven't had a problem since.
The "light out" display in the dash monitors running, brake, AND license plate light bulb integrity.
__________________ Officer: "I see you set the table for four." Groucho Marx: "That's nothing--I set my alarm clock for eight."