Is that a sedan or a wagon? I had a similar problem with one of my brake lights on my sedan--other lights in the fixture worked fine. Not the fuse. Replaced the bulb. Replaced the circuit board--no dice. It turned out to be the white bulb holder/socket, which is difficult to test because they're not compatible with any of the other ones on that fixture. My mechanic pulled one off a parts car that worked just fine.

What do your lights look like under the plastic covers? Some people get rid of the bendy circuit boards and just wire everything directly from the harness connector. You might actually consider doing that because it's less problematic than the circuit boards.