The water can either get in from underneath, from the wheelwells r from draining down through the body. I don't know how the W210 sunroof drains but that could be a source for example. If there's any dirt or debri in the channel the least amount of moisture ,even humidity -tempature changes, can cause muck/mud. Then it's just a matter of time before it doesn't drain at all. I have always made it a practice of clearing them out when washing the car. Granted I live in an area where the winters are harsh as they are in the north so it's a bit easier to do year round

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1983 300SD
1988 420SEL