On my 1987 300TD wagon my instrument lights are working and dimming just fine - but all the other interior lights are out.

It is acting like that dimmer is bad, but I know it is not.

I had been moving the center console around a bit and thought I may have knocked something loose, but I can't see anything disconnected or frayed.

Any ideas where to start?

Do you mean all the other lights on the shifter, climate control, window switches are out?

If so there is a module called N40 behind the cluster. It repeats the dimmer signal to all the other lights. The instrument lights are direct from the dimmer.

There is a terminal block in the console for the light distribution.

I would get hold of a wiring diagram.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

My 87 300d had a burned trace on the cluster circuit board just after the dimmer. The cluster lit up, but HVAC control and center console were downstream of the burned spot. I soldered a jumper wire in around the burned spot and it was good.

I have the CD with the Maintenance Manual.

According to page: 00-B.2/6

It says that "N40 Audio power amplifier (circuit 58D)". (I just do not understand why they designated it as this)

As far as I can tell N40 is a relay just like you said. It does seem to be the amplifier for that rheostat/dimmer. I will see if I have an extra one and replace it.

Where can I locate a real wiring diagram?

The one on the FSM CD is "The" Real One.

Oh! You mean an Enhanced/Zoomed PAPER Schematic Diagram?

Y'all Kinda have to Manipulate the Data and Pick which "Zoom"
Y'all wanna print.

'Have an 8-10 Year old Heir, or Niece or Nephew?
(Neighbor's Budding Genius will do Also.)
[Desperation is throwing yourself on the Mercy of the Local P.T.A.
They'll have some little Wizards that can help you.]

Hand them the CD,Tell them what you require,And Strike a Deal.

__________________
'84 300SD sold
124.128

I know that there has to be something more than what is on the FSM CD's that I have. I have seem some online photos from time to time that show what looks like a regular schematic.

I have the updated directory structure for the FSM and do not see what I know I have seen before.

Maybe what I have is not the complete FSM.

Like this kind of diagram:
http://www.mercedesw124.com/wp-content/uploads/2013/01/Wiring-Diagram-Mercedes-Benz-W124.jpg

I do not see these on the FSM CD's that I have.

The "N40" designation for some reason got translated as "audio power amplifier".
To make it worse, the later 124s have an N40 in the trunk that is the audio amp.

The N40 for the lights is behind the cluster. It's a metal can that looks like an oversized relay. It does have a transistor circuit in it that takes the variable voltage from the rheostat and provides a replica signal for the rest of the interior lighting. Maybe that is how the translation happened.

The best wiring diagram out there is at w124-zone.com. You'll have to register but it's free.

They have a complete copy of the FSM as well as the wiring diagram in North American format. Note that a couple of the pages did not make the big PDF, they are located in a drop down menu below the link for the main wiring diagram.

This manual covers all 124s up to the mid 90s. If you have access to Adobe PDF or a 3rd party tool that allows you to add and delete pages, do yourself a favor and make a subset PDF that only has the pages relevant to your car. Then put it on your phone/tablet and you will always have it at the ready for troubleshooting.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

After messing around endlessly with the N40 module, I found the console lights were out because of bad wiring in the rear door hinge ... the illumination for the rear window switches is on the same circuit as the rest, and the wire was shorting against the body of the car.
If it is the N40 or the door wiring, there should still be some lights working. Lights out would be shifter and antenna, rear headrest, rear dome light and window switches and I believe the center and right vent dials. I can't remember exactly but I'm pretty sure the CC button illumination did not go out in my case.
If those are the trouble lights, probably worth the look at the door hinge wires ... very common for them to be in bad shape. Mine had wires totally severed and poking out.

__________________
1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)

To the O.P. :

Whatever you've "Glomed Onto" in the "Representation" of a Mercedes Schematic (Wiring Diagram) in your reply #7 IS NOT A MERCEDES PRODUCT !

'Much Better y'all follow the directions of reply #8 which directs y'all to the
"Tried and True" Version of the FSM 's "Tweaks",Which exemplifies the BEST
of "Translation" of Mercedes original Information.

__________________
'84 300SD sold
124.128