I've noticed for many years that, compared to other cars around me at the same time, 123's and 126's are poorly lit, especially when overtaking them in less than clear conditions. They are harder to see than other cars under the same conditions. Those two, "dinky", little tail lights do not seem to be adequate - maybe, I've gotten used to seeing so much more light from so many other cars. Because of this, several weeks ago, I "activated" the "fog" positions in the tail light assemblies on my 123 to operate as TAIL lights; 'just did the same thing to my 126 This afternoon.
I acquired a tail light lamp holder assembly from a salvage yard, removed the individual lamp holders from it and installed them in the unused "fog" positions. Wires from the "new" lamp holders are spliced into TAIL light wires near the original tail light bulbs. I now feel much better about having significantly improved rear lighting on both of my MB's - both with FOUR tail lights each using identical, original wattage bulbs.
BTW, individual lamp holders within the assemblies on 123 and 126 are identical - possibly other models, too; you don't HAVE to get a 123 lamp assembly to get holders for 126 and vice versa, etc.
Salvage yard asked me to take a lamp holder assembly from a wreck with a broken or missing lens - I did - I didn't need a lens.