Not so much a matter of them being plugged, but that the path for the water is straight through to the outlet, as opposed to a leak in the tube/pathway somewhere (rust, eg.) allowing it to exit inside the body (inner rocker panel, eg,). Perhaps someone who has had hands-on experience can clarify if the drainway is a steel tube, or a non-metal (plastic, vinyl) tube. If steel, I can see how a partial clog could allow water to sit long enough to rust through a small hole allowing water to get diverted into the rocker cavity.

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1986 560SL
2002 Toyota Camry
1993 Lexus