It can't be caused by the crack in the plastic window. My car has a small crack in the little right-sided plastic window as well and we have a lot of rain in the Netherlands! I have no water leaking problems at all!

If I were you, I would remove the passenger seat and start teasing the car with a hose while someone sits inside the car.
After the seat is removed, you can also remove the doorsill and lift the carpet.

There must be another cause for the leakage (either the drainage of the soft-top compartment trunk or along the door rubbers or from behind the door panel (leaking water-resisting plastic).

good luck, greeting & don't give up!

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)