About 3 years ago, my 2013 BD LS had a couple of the bulbs go out on the 3rd brake light. It seems like it had 20+ bulbs, so it wasn't a problem. My inspection was due recently, so I asked someone to check if the light was working. It wasn't. When I checked, not only wasn't it working, it had water in the fixture.
There is another thread where a few guys replaced a diode and got it working again. I'm not that handy so I opted to buy from GM. The best price I could get was $300. Some websites sell similar fixtures for less, but I wanted to keep it simple.
After I got it, it really wasn't hard to replace. However, I bought some silicone and tried to limit water wrecking this one.