I'm not sure this will be helpful, but the set up described on the passenger side is the correct way for it to function.
The taillights on most German vehicles are cross-wired and configured so that if the main brakelight fails, there will be a backup brake light.
So the problem is that your normal left brake light (upper one) is not getting any power which is triggering the fault and thus prompting the bottom light to act in its stead......all I can say is make sure there's no corrosion and the bulbs are good.