How to test 86 300SDL front dome light
 

How would I test the front dome light on my 86 300SDL to determine if it is functional? I tried different combinations of 12V vs ground on the five pins and changing the switch settings, but I never got what I would consider a normal result.

Your drivers manual will have the expected results for each switch setting.
If you don't have the manual let me know and I'll go make a copy of mine (lighting function page).

IIRC - Two manual "on" settings, one auto on/off with fade off feature setting triggered by opening and closing the door, and an "always off" setting.

When I plug the dome light unit into the connector I get no lights on any setting with any combination of door or switch positions.

What I am trying to test is the front dome light unit itself. Is there a way that I can apply 12V and ground to any combination of pins with any combination of switch settings to verify operation of the light/circuitry in the dome light itself?

Hate to sound dumb but have you verified the bulb is good?

Just because it looks good doesn't mean it is.

All five bulbs have been tested separately.

Ok fair enough

I don't have the diagnostic for a 126 second generation handy but a lot of things are shared with the 124.

I think they share the same dome light assembly with the slow fade and the airplane style flashing seat belt sign.

Here is the info from my 124 wiring diagram:

There are 5 pins on the light assembly:
1: circuit 15 (live with key in position II) from fuse 5 on a red/violet wire
2: TK (türkontakt) door switch input - ground to activate - brown/gray wire
3: circuit 30 (always live) from fuse 9 on a red/white wire
4: circuit 31 (ground) on a brown wire
5: circuit 15 output to vanity mirror lights - just a pass through from pin 1 on brown/red wires

In order for the light to work you need to apply power to 1 and 3 and ground to 4. Apply a ground to 2 to turn on the main light and remove the ground from 2 to begin the fade out timer.

I would get a meter and check the connector pins for the correct voltages.

The symbols mean:
0 is off
Book is reading light
Half moon is on while doors open
Half moon with sun rays is on always

I don't know if the circuits are the same, but on my 190D, the dome light doesn't work if the fade circuit board has failed. The circuit board can't be separated from the lamp, so no repair is possible. Rather than replacing the entire dome light, I shorted the circuit so that the fade board was bypassed. With this shunt, everything works as documented, except the fade.