2012 W900 left brake light

[Mikanic]

You have 2 power pins at connector A (black), 4 power pins at connector B (white) and 2 power pins at connector C (blue). I dont remember the exact pins. You should get a fault from chassis node if your missing power but to know which power circuit you would have to check faults with ESA.
I hope you started your diagnosis by checking the fuses under the hood.