Originally Posted by jay_bob

Anyway I finally got a chance to download the diagram and take a look.

I'm fairly new to Mercedes and all my time so far has been on 123s but I think I see what is going on here. The 123 electrical manual spans about a dozen pages and it is crazy simple compared to a 124. I hate to think what's going on in my wife's 2012 Honda Pilot

Issue #1, no dash lights
I think the N40 relay is like a 'rheostat repeater'. It takes a 12 V reference signal on pin 4 (58d) from the dash rheostat. The items on 58d (mostly on the left side of page 130/3) are directly off the rheostat.

The reference signal on pin 4 has 12 V on it when you have your lights on because you are getting good voltage there, and you said your main cluster lights were working.

Pin 2 (58D) is the output. N40 takes power from pin 1 and electronically regenerates a signal on pin 2 that is a copy of what it is seeing on pin 4. I suspect they did this because there are too many bulbs for the dash rheostat to handle by itself (unlike in the 123, everything is off the rheostat, but I have about 10% of the bulbs you do). They wanted to keep the critical bulbs direct off the rheostat (instruments), but the less important items got relegated to N40.

When you tried to tie Pin 1 (12 V from fuse 9) to Pin 2 (the remainder of the lighting circuit that is not working) you blew fuse 9. That leads me to believe, as you do, that you have a short in your lighting circuit.

Unfortunately it is extremely difficult to use a multimeter to find a short in a circuit with incandescent bulbs. The cold resistance of an incandescent filament is nearly zero; as the filament heats up, the resistance increases to anywhere from several ohms (brake lights) to several hundred ohms (dash lights), inversely proportional to the wattage of the bulb.

Here is a trick to try:
Go to the parts store (or look around in your parts stash) for an automotive bulb socket with wire leads and a 12 V bulb. You want a bulb that is normally used for brake or turn signals. Wire the bulb socket across pin 1 and pin 2 of your N40 socket. This puts the bulb in series with the remainder of the lighting circuit. If you have a downstream short, the test bulb will glow brightly. The advantage is that the bulb limits the current, so you won't blow the F9 fuse while you track down the short, and it gives you a visual indication of what is going on.

Unfortunately you are going to have to bite the bullet and pull your console so you can lift wires at X6/1 one by one to find the shorted one. When you remove the shorted wire, the test bulb will go dim or all the way dark.

That damaged wire pack in your B pillar is certainly suspicious. There is a wire from the output of N40 in that wire pack that goes to the window control switch in the left rear door. (Was this the car that got hit in the left side?)

Once you get the short fixed, you can temporarily run without N40 by jumping pins 1 and 2, the lights will just be at full brightness. If you smoked your N40, try cracking the case, strip out the guts, and solder a wire between pins 1 and 2 for a good temporary fix. The fuse is 8 Amps, so #16 wire is plenty sufficient.

Issue #2, weird tail lights
That is a classic lost ground situation. Fix the bad ground connection and you will be all set.
What is happening is that your left side brake and tail lights share a common ground point. Normally the power for the left side tail lights flows like this:
fuse -> head light switch -> tail light -> ground.

And your brake lights go: fuse -> brake pedal switch -> brake light -> ground.

In both these circuits, the ground is the metal frame of the tail light assembly. There is a wire that brings that frame back to the actual ground on the body. I think you are on the right track that the wire is loose and probably needs to be reattached.

When you lose the left side ground, the power tries to get to ground any way it can. Now the power goes:
fuse -> light switch -> left tail light -> shared ground that isn't really grounded -> left brake light bulb -> brake light circuit (which is at 0 V since you aren't on the brake) -> right brake light bulb -> right side ground.

Your bulbs glow dimly because you have effectively put 3 bulbs in series. The brake bulbs are higher wattage so they glow brighter than the tail bulbs.

When you step on the brake in this condition, you bring the brake light circuit to 12 V. This causes the voltage across the tail light bulb to go to zero, because both sides of the tail light bulb now have 12 V on them. This makes the tail light go out when you step on the brake while the brake lights illuminate normally.

Originally Posted by jay_bob

Yes I would suspect the N40 has failed shorted.

The N40 is basically a current amplifier and a short on the output stage would send direct battery voltage right to the output.

What happens when you remove N40 from its socket?