Somehow you are going to have to get that carpeted panel off the lower half of the back door to access the mechanism.

Normally you would open the door and take off the plastic trim cover on the bottom edge of the door, that is held on with about a dozen plastic pin fasteners. That frees up the bottom edge of the carpeted cover. I think it's hooked at the top with some clips to the framework of the door.

But since you can't get the door open that is going to be interesting. Maybe you can force the panel upwards enough at the top to get it released from the clips. Hopefully without breaking anything.

Once you get the cover off then look at the service manual, there is a procedure for adjusting the locking mechanism.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech