All my interior lights stopped working on my 1983 (W126) 300SD. I didn't really notice when it happened but it soon became time to find out what's going on.
I checked the fuse - fuse 15 - and sure enough, it was blown. Oh great, this is an easy fix then - simply put in a new fuse - *poof* - and another fuse was vaporized. Oh joy. A short someplace.
Looking at the schematic, there's two wires that come off of fuse 15; one runs the front interior lights, one runs the back. There's all sorts of other crap hooked in there twoo like door open switches, a time delay for the interior light to go off and so on and so forth.
Reasoning that if 12V is shorted to ground someplace in these two circuits the way we approached this was to test for conductivity between the +'ve and -'ve terminals of every bulb socket and chassis ground. They should show conductivity to ground on one post of the bulb socket and infinite resistance on the other - expect one that's on a shorted circuit - that one will show conductivity to ground on both terminals of its bulb socket.
Turns out in my case it was the visors. I'd heard if you clean the metal clips that turn the visor lights on and off (look at where the visor clps to the body on the end opposite wo where the visor is permanently attached to the roof) but that didn't help.
Upstream from that is the front dome light that also contains the time delay relay. In my case it was fragged and the cause of the short. It appears what happens is this: there's a signal from the door open switch that is a trigger input into the dome light circuit. The circuit redas this and the switch position and if it decides it wants to turn on the light it activates a solid state relay on the small circuit boars insode the dome light and turns the light on. I was tempted at first to just wire the door open trigger as a source of 12V to light the bulb and forget the time delay and switch - having the light come on when the door is open is better than nothing but then I quickly realized I don't really want to dissipate 5 watts - the bulb rating - through the poor door switches. Bad idea. A new one is under $50. If you're brave and want to fix yours, read this link