I don't have a W126, mine is a W201..but should be the same.

The front & rear dome lights are on separate circuits. My model year did not have a 15sec delay like the later year models.

I used a feature from my aftermarket alarm to turn on the front & rear dome lights, when the alarm is turned off & doors unlocked.
The lights stay on for 1minute & turn off...enough time for me to walk to the car & get in. The lights would turn off immediately as soon as I turn on the ignition, even if the 1minute is not up yet. Also I used two diodes to isolate the front & rear circuits, to keep them separate like before.