On the dash of my 420 SEL, W126, yr.1986, there is a rear dome light switch, which normally turns on the rear dome light and both rear door lights. Problem is, it doesn't do that anymore. It now only turns on the right rear door light.

You'd immediately conclude that the lamps in the rear dome and left rear door lights need replacing. Not so, however, because
when I open the left rear door, all of the interior lights in the rear come on......as they should.

Then another puzzle arises, because when opening the right rear door, the only light working is the right rear door light.

In reviewing the wiring diagram, I keep thinking there's some problem within the rear dome light switch, on the dash,..........in spite of the fact that the switch will turn on the right rear door light, while not turning on the other lights. The down-side leg of this switch attaches to the right rear door light switch and makes me wonder if somehow it's not picking up the necessary ground for the dome light and left rear door light.

I've tried to remove the dash mounted switch (000-820-59-10) but whereas it fits into a base socket, with two pens, it has a small black wire that's hard wired, which isn't shown on the wiring diagram and I can't discover from whence it comes.

I've searched the "Shop Forum" archives for three days but can't find a similiar problem. It could be that I just have a broken ground wire someplace.

Suggestions would be appreciated.
Thanks, William