If it is a W124 you can just pull the doorsill off (no need to be that careful). It is clamped on the threshold with a couple of u-shaped clamps.
If it is a W124 you can try to adjust the trunk lid lock a little bit more to the outside and it may help getting rid of that intermittent problem.

greetingz,

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