It can also be the N7 bulb monitor relay failing.

I had this problem with my right front marker filament in the turn signal. Fortunately I was able to open up the housing and find the cold solder joint.

This part is very common in the 124 and gen 2 126 so I always grab them when I am in the yard. It is the long relay in the back of the fuse box and has nearly 2 dozen pins so it's in there tight.

Are you getting voltage at the connector for the tail light assembly? If you are not getting voltage I would suspect the N7.

__________________
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