I will respond to this thread starting at the first post. I too didn't see any posts except this one. Surely there must be a bunch of folks having trouble with this pesky lighting they put in the 210 series! Being a fan of the W124 Hellas with GLASS fronts and connections made using silver plated copper posts, as I dig into the issues I have with the 210 headlights I am sad that MB quality has dropped so much.
The first experience I had with headlights with plastic fronts was on a ML320 belonging to my BIL. His plastics were so fuzzy they weren't attractive at all and I am sure they weren't passing all the light form the bulbs to the roadway so I bought some stuff to polish the faces and he went at them and they looked a lot better for about a year, slowly they got fuzzy again.
All along I have a issue that was lurking on my '99 E300D. The PO had installed Xenon headlight lights from a later 210 car and he worked and lived in the Mohave desert with its occasional sand storms so he placed a plastic film over them to protect the plastic. When I bought the car 6 years ago it already was getting brown along the edges.
I peeled off the crusty top where it separated from the headlight so I can't share the ugly looking results but I advise anyone thinking of using that stuff to forget it! Basically it ruined the lens assy as no amount of polishing could have repaired the thick plastic sheet that adhered to the faces of the headlights! The gooey adhesive turned to hard solid crud strongly attached to the plastic lenses! Ii just wanted to leave it as is. TO live with the mess!

Then fate creeps in!
A few days ago my wife's niece who is the principal driver of the car came home late at night and along the way she was stopped by the local police who claim her headlights weren't bright enough! She got off with a warning fortunately no "fix it" ticket!

So faced with this, today I pulled out the set of nice looking 210 headlights I purchased a few years ago with the thought of swapping the into my '99. Seemed like a plan - easy enough.

Cheaper than new!
For example my prices for "Composite Assembly Lighting Unit" (aka Headlights) from a dealer with a small discount (thats all anyone can hope to get) are:
210-820-4461 Left with Xenon Lights $605.
210-820-3761 Left w/o Xenons $235
(Right side same prices).
Obviously the ballast is responsible for the huge difference in prices. And I can't find a headlight assy alone without the electricals. And these have an electrical aimer in them which the W124 headlights didn't have so there is some technology involved and that adds to the cost. In all fairness I should admit the 124 Hella Euro headlights on my '87 300D have the vacuum actuated aimers but the rubber diaphragms cracked and no longer hold vacuum so I disconnected the vac lines. I wasn't using the aimer anyhow, after buying the bezel that goes around the light switch and running vac line to it and all, just an exercise.

Back to the 210 dilemma

So now I needed the headlights I had been putting off using so I removed them from storage today and popped off the back covers and immediately discovered the insulation on the wiring all flaking and gone!
I showed this to a friend who operates an Indy garage and the consensus of opinion was first that MB quality has gone to the dogs (but that brings him lots of biz) but more importantly I couldn't use the spare headlights even with their almost new looking plastic lenses because the electrical connectors are impossible to R&R. Even if the wiring was available its such a nightmare inside.
But the lenses could be popped off as they are only held on with a bunch of tabs that snap under clips molded into the housing.
So after some prying and cursing I managed to get the lenses off. Photos to follow.
I'll report back after I get the existing headlights out and swap the lenses onto them.
Someday someone else may want consider doing this in lieu of spending a lot of $$.

My wife can't believe the cost of new OE headlights so I showed her the MB logo on the estimate and told her thats how they make money, selling cars near a loss then making it all back selling parts.
She would rather get rid of the car than put $1200 into it and I agreed no way would I go that route and I showed her my $110 investment for headlights a few years ago is about to pay off, although with some more wrestling with the car.

I just hope I don't discover flaky insulation on the wiring in the headlights on the car but its not here today so I'll have to check and report back tomorrow or this weekend when I plan to roll up my sleeves and finish the second half of this job.
DDH

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!