what 007ben said about both license plate lights working. If one is burnt, you'll see the message on the OBC.
I also had a problem with "brake light failure" and license plate light failure messages, and it was a problem until the bulbs were replaced with OEMs.
I also had a problem in my '97 328i that might be relevant to your situation: the lights wiring harness that runs onto the trunk assembly, although not appearing to bend or bind, contained wires that were bent and brittle. The dealership "patched it" for me and said it would be about $600 CDN to replace the wiring harness. Labour mostly, I assume.
So it all works great for now, but I see more $$$ on the horizon.