I have an87 300E. The dome light switch is not illuminated on mine. The owner's manual gives instructions on how to replace bulbs ('push the lamp assembly to the right, lift off the cover on the left side with a screwdriver and pull the assembly out').
However, the only bulbs they refer to are the 10 watt dome light, the 5 watt reading light bulb, and the 1.2 watt seat belt reminder bulb. No bulb for the switch. Funny, they put an LED in the radio fader knob, but nothing in this switch.
Hope this helps - I don't know if your 92 is the same.
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E