Tail lights on the modern MBs are on the rear SAM. They use a single filament (or LEDs for the really new ones). Our favorite friend PWM (pulse width modulation) is at play again here. Applied voltage is always 12 V, but it gets switched on and off rapidly to make the light appear dim for marker light mode. Brake mode gets constant 12 V dc.

If you’re having trouble and you’re sure it’s not the wiring or the socket it will be the SAM. They use a power semiconductor to switch the voltage to perform the PWM. I suspect that this is a readily available MOSFET that can be replaced easily.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech