The wiring diagram for the car is on startekinfo.com, workshop resources, go in a couple pages to find the 123 chassis entry in the table.

Not much going on electrically with a 123 so they are easy to troubleshoot with a set of prints. No computers or any kind of can-bus vodoo magic, everything is a straight wired circuit.

I would disconnect the battery then loosen the nut on the face of the fuse box. Then drop the fuse box behind the dash. The wire pack has enough slack to let the fuse box lay on the floor board for testing purposes.

On the back of fuse 1 there will most likely be several wires. Disconnect one of them and leave the others connected. Hook the battery back up, see if the short went away. If not try lifting another one of the wires until you find it. Then go tracing down that wire to see if there is any damage.

Its all detective work from here...

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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