W123 dome light works by grounding the lead coming from the light to the door switch.

The driver side switch lead passes through the seatbelt/headlights on/key in switch warning box. There is a coupling diode in there that isolates the left side switch for the logic in the box so that the buzzer only sounds if the door is open, key off, and either lights on or key left in switch.

I would check to see if someone unplugged the buzzer box. That would stop the left switch from turning on the dome light.

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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