Before tearing the car apart, try this trick:

Check the top piece that the belt slides over before going behind the panel. Over time dirt and grime transfers from the belt to the sliding piece and it cause the belt to hang up. Clean the top piece and see if it helps the belt performance.

Most 123s do not have the ETR (emergency tensioning retractor). But if you have a late 123 with driver air bag, then beware you will probably have this. In this case you need to disconnect the battery and wait 10 minutes before removing the red connector from the ETR. The ETR will be a 6 inch long cylinder on the reel assembly. It has a small pyro inside that releases spring tension and sucks the belt in tight during airbag deployment.

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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