Same problem here. Anybody know?
The service manual says the whole assembly needs to be replaced, but only one ~3 inch segment is dark on mine and I'd prefer to repair it.
Especially because the part is $240+, vs. a bulb or fixing the circuit board.
Genuine BMW Third Brake Light Assembly 63258362039 - Free Shipping
I've already removed the light assembly, which is straightforward in the service manual (and way more involved than expected - step one is to remove the spoiler).