Hi,

I assume all rear lamps except license plate ones are LED's and there are no fuses for them. Your high mounted brake lamp is more likely gone, you will have to replace it.

 

Try using a trouble light continuity tester. Put it to each fuse both sides forget the diagram and see if it lights. If all the fuses light you have a dead 3rd light.

 

First step would be to check wiring harness at left trunk hinge. Picture shows 211, 219 is the same.

If it is found lamp replacement is needed, part # is 2198200156 removal:
http://workshop-manuals.com/mercede...ounted_brake_lamp/component_information/technical_service_bulletins/page_17299/
http://workshop-manuals.com/mercede...ame/interior_moulding/trim/rear_shelf/component_information/service_and_repair/ + next page
http://workshop-manuals.com/mercede...and_repair/ar68.30-p-4020tx_remove/install_footwell_paneling_(left)/page_15181/
http://workshop-manuals.com/mercede...ion/service_and_repair/ar91.10-p-3310tx_remove/install_seat_cushion/page_16466/
http://workshop-manuals.com/mercede...ndows_and_glass/windows/window_shade/component_information/diagrams/page_17777/
http://workshop-manuals.com/mercede...rvice_and_repair/ar68.30-p-3445tx_remove/install_rear_speaker_cover/page_14433/

 

Konigstiger I just did the same repair. Incredible design wires break from metal fatigue. Don't know if my tail lights were out but when my trunk stopped working I opened the rear driver side trunk and found eight wires " broken". Cut out the old wire replaced it with new and then used connectors to reattach. Not really pretty but fixed the problem. And it only costs $100,000 to have to deal with poor engineering. Lol.

 

Thanks for the reply Koningstiger!
CLS has third brake light on the deck, at the bottom of rear windscreen, so wires to it do not go via left hinge. Only wires for numberplate lights. Must be some other wiring that is knackered.

 

Right, here's the wiring diagram.