I have no real solution for you but if you pull the fender off you will have direct access to the drain tube. A bit extreme I know.. but you will also find lots of dirt/mud/leaves/sand perhaps under the fender.

Try shoving something up the tube where it exits the car... I can visualize it on a w126 but not a w124.

goodluck

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08 R320 CDI current

Past
95 E420
87 300D Turbo 5spd
90 300TE
83 300SD
85 300TD
92 400E
85 190D