Just recently swapped out the headlamp rear access hatch from my kid bro's 190e (Euro) for the same reason.
The original hatch had a crappy gasket and the hatch did not seal very well.
We took a hatch off a spare headlamp unit, and there has been much less incident of fogging on that lens. The other lens doesn't fog at all. This leads me to believe that it is supposed to be a tight fit and that there are no vents...I could be wrong but it would seem logical...
The weather here in Singapore is VERY humid and I don't get any fogging at all on my headlamps (190e) so I could be right about the tight fit.
Babybenz 1991 190e 1.8