Use an 'automotive test light' the thing that looks like a screwdriver with a 12 V *incandescent* bulb in it (not an LED) and has a clip lead coming out the back.

Put the clip lead on the +12 V terminal on the battery and probe pin 1. The light should light up brightly. If not you have a bad ground.

When chasing bad grounds, use the test light to provide the source and load and let the ground path you're trying to trace complete the circuit.

You can also use a surplus bulb socket with a reverse light bulb (1157 or similar) and do the same thing. You need one that brings both sides of the bulb out to 2 wires, not a single wire plus the shell kind. Put the bulb between battery positive and pin 1 on the relay socket. That will actually load up the circuit enough that a bad ground will become apparent.

Also on 124s don't overlook the OVP relay, the fuse holder clips can go bad on the top and that wipes out pretty much all the engine electronics.

__________________
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