In my 300SDL, all of the light switches do light up, they're just "dim" unless activated. It struck me as odd that 2 of these 3 switches were completely dark.
I pulled the switches themselves and removed the switch end of the light tubes (fibre optic line). With the headlights on, these lines don't light up.. definately a problem at the octopus.
I'll dig into the dash tonight and report back. It is sounding a lot like the 2 fibre lines are disconnected at the octopus OR the bulb in the octopus is out. I'm still wondering why the dome light switch works.. perhaps it has its own bulb?
'83 300SD 335,000km (207k) mi
'87 560SL 163,000km (101k mi)
'86 300SDL 356,000km (220k mi)
'92 500SEL 250,000km (155k mi)
'90 300SL 140,000km (87k mi)
'01 S430 260,000km (161k mi)
'03 SL500 167,000km (104k mi)
'07 GL320 CDI 4MATIC 289,000km (162k mi)
'07 S550 4MATIC 170,000km (105k mi)