I had the same problem when i bought my '90 300E back in July....the rear dome light would only come on with the dash switch....being super anally-retentive, i just had to fix it. Interestingly, the only time which it did work was after having driven in a monsoon. After pulling out lots of wires and carpeting and cursing having bought the stupid car, i finally found what was wrong, which of course was the last place i looked. Underneith the front-passenger carpet and under the plastic shield, there is a big multi-plug that connects the wires coming from the back of the car to wires coming from the front of the car. I opened up the cover of the female side of this multi-plug and i discovered a resistor/connection that had shorted out. So, I soddered the connection back and then the rear dome light started working when the rear doors were opened. Later on, i found out from somewhere that there are a couple holes in the firewall for some reason or another above the passenger footwell area, and i am thinking that during hard or deep rain, water can seep through these holes and run down to the multi-plug and that is what shorted the resistor/connection thingy. Hope this helps -- MIKEY