I have a similar question too about these rear light's. As I understand it ( and thats a bit of stretch of the imagination .. Veh electrics, and especially modern CANBUS systems are way outside my comfort zone....in spades !! ) when the parking lights are on, and you press the brake, it just "sends" additional current through the park circuit cables, causing the LEDS to brighten, which gives the brake light. OR are the park lights and brake lights on their own individual and independent circuits?MB started using part-LED rear cluster lights from around 2010, though it was phased-in at different times for different models so not sure when the ML got them.
But it's very likely that the rear cluster is part-LED. Which light failed? Rear light or brake light?
If the failed light is LED, then the official solution from MB is to have the cluster replaced, though unofficially they can sometimes be repaired by soldering replacement LED.
Also, check all the electrical connectors for corrosion etc.
In my previous 2010 E class w212, I used a universal relay to wire a car trailer to the car electrics. No problem's at all...worked like a dream. But now with my newer 2014 E class w212,, its problematic. Left / right turn, no problem. Brake light. no problem. But park lights? Yes, because in the 2010, brake /park were two different feed's and so no problem, but in the 2014 the brake / park are one and the same,,as you point out its the Pulse Width Modulation. BUT..one hope for a solution..if I take the trigger for the bypass relay from either the Nr plate light, or from the the same connection the SuffolkSifter is having the problem with? In his case, the brake light does not work in that cluster, only the park light, yes? I'm open to suggestions / comments / advice etc. I just need to get a constant park light feed to the relay for my trailer.If they are the ones that brighten then they are on the same circuit.
For sidelights they are turned on and off very quickly with a short pulse. For brakelights the pulse is longer.
Known as Pulse Width Modulation
By the 3rd brake light, I take it you mean the rear window one??? And keep as far away as possible from the rear light clusters ? ( Indicator's being the exception ) Good Idea !!!@Justintyme , that is exactly what I would do - take the sidelight trigger from the number plate light.
I would also take the brake light trigger from the third brake light as it is much less likely to be modulated than the brake lights (you can often see this flicker if you look out of the corner of your eye)
Can’t promise 100% that this will work as it’s 40 years since I was an auto electrician