I know the early 124s do not have a master door connector. The body harness extends through the hose and terminates at each device in the door directly.

Unfortunately the engineers chose wire with a low strand count to cross over the hinge point. It will have something like 7 strands of fairly heavy wire to make up the required cross section.
The proper way to do this is use wire with many fine strands so that it does not experience metal fatigue as the door is opened and closed.

The best way to fix is to obtain some wire with a high strand count and splice a piece in where it goes through the hose. This is not easy as you're having to work in tight spaces.

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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