The only soft light in the first gen S124 was the front dome light (between the visors). Not sure if the later models ran the rear dome lights through the convenience control module under the left (at least in US) rear passenger seat to effect a soft dim. The convenience control module did not make an appearance until the early 90s in the US, so I have never seen a car with one except in the junk yard.

I'm assuming you've isolated it to the dome circuit by placing an ammeter in series with the fuse?

I measured my standby drain on my S124 for another member a couple years back. There is so little standby drain on these cars that you literally can detect the clock ticking every 20 seconds. About 15 mA IIRC, and probably most of that drain is due to the aftermarket cheap Chinese remote unlock receiver that I installed. The clock movement draws about 50 mA for about a second every 20 seconds.

__________________
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