Common faliure. You need a new check strap.

Yes, especially on the rear doors which tend to sit closed a lot more.

This is very interesting to me, as when you go to the dealer to buy
these units you will see one of the upsides of owning a older mercedes.

Unlike any other manufacturer I have ever dealt with (and I have dealt
with a few) the replacement part the dealer sells you is actually
re-engineered to fix this issue.

The original part had a design flaw, which meant that the check ball
would allow water to pool around it and eventually this would rust it
to shot(causing very difficult door opening/closing).

The replacement item your dealer sells you (about $28 here in Seattle),
is covered in a plastic sleeve to prevent this.

I had to replace both of these on the rear doors of my 1986 190e when I
bought it,  but the drivers door was still OK, and the front passenger
door is hanging on too...

Marty