The brief
The E200's headlamp had gone yellow and hazy, the lens cloudy and dimming the light, with moisture building up inside the housing. He asked about it, and the honest answer is that a polish-and-recoat job on a headlamp like this is only ever temporary. The lens has a protective coating, and once that breaks down the plastic underneath yellows and clouds and scatters the light, so the beam goes dim and patchy, which is a real safety issue at night. Restoring the lens removes the old coating and the discolouration on the surface, but the new coating fades again before long, and it does nothing about the underlying problems, the moisture getting in and any damage to the housing. The lasting fix is to replace the headlamp with a new unit that has its coating and seals intact, then align it properly.
The diagnosis
A check confirmed it, the headlamp lens was yellowed and crazed with its coating broken down, and there was condensation inside the housing pointing to a failed seal. A surface restoration wouldn't have addressed the moisture or lasted, so the sensible fix was a new unit. That's a headlamp replacement, a complete new unit, fitted, connected, and aimed correctly so the beam pattern's right.
The work
The old yellowed headlamp was removed, and a new genuine Mercedes-spec headlamp unit fitted, connected up, the bulbs and adjusters transferred or fitted as the unit required, every fixing torqued to spec. The new headlamp was then aimed on the beam setter so the pattern and the cut-off are correct, and the lighting checked all round. A check confirmed a clear, bright beam with a clean pattern, no haze, no moisture, and the lighting working as it should.
The outcome
A clear, bright headlamp with a proper beam pattern, no yellowing, no moisture inside, and the lighting aimed correctly. The E200 went home with the headlamp sorted for good. A polish would have looked better for a while and then faded again, so replacing the unit fixed the dim beam, the moisture and the cosmetics in one lasting job, the right call for a part you rely on at night.