The one depicted in Texholdem's post is pretty much the only mass-produced cup holder I've seen produced for the W124...and there are a number of sources for it.

Beyond that, you can custom fab one yourself. Personally, I stuff my drinks in the console, and slide the door against the cup to hold it.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car

Yeah, cup holder is quite an issue for German cars in general. I've been living there for a long time and it seems to me like a cultural matter. Germans do not tend to eat and trink a lot while driving.
I always had German cars over there: a Beetle, 3 Opel Record, 2 Audi, a BMW 633CSi, 3 MB but cannot remember ever having a decent cup holder.

I recall a time even in movie theaters there were no cup holders at the seats like in the USA. You yourselves are the cup holders .
Perhaps they are catching up with the Amis (German abbrev. for Americans) now.

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1996 E320 since 1/16/08, 171K miles as of Feb 2011

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1989 300CE - R.I.P. Dec 29 2007
Other MBs (sold): 1992 300E-24 - 1979 350SLC - 1984 230E - 1990 300CE

I bought one similar to that one, except more expensive, because it was a burlwood one to match the car's wood. I cut a cork template for the bottom of it, for when it was in use and would recommend doing that..

Having had it for 10 years, I can say it is awkward if you are right handed, and note they are NOT adjustable for different sized cups. The common 16 oz plastic water bottles fit right into them.

It is true that Mercedes Benz resisted the cupholders in their cars until they couldn't resist the pressure from American buyers that wanted them. I read a survey that said that was the most important option desired by Americans.

Driving is a serious business in Germany, cupholders never fitted in their automobile culture.

Some American made minivans had 17 cupholders in them. The latest Chrysler cars have heating and cooling features in their cupholders if you want them.

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1991 560 SEC AMG, 199k <---- 300 hp 10:1 ECE euro HV ...

1995 E 420, 170k "The Red Plum" (sold)

2015 BMW 535i xdrive awd Stage 1 DINAN, 6k, <----364 hp

1967 Mercury Cougar, 49k

2013 Jaguar XF, 20k <----340 hp Supercharged, All Wheel Drive (sold)