Might also be your N7 light monitor relay has taken a dump. That happened to me and that is how I lost a marker light that I could not for the life of me figure out what happened.

First make sure your fuses are good. Fuse 13 is left low beam and fuse 14 is right low beam. The fuses are located past the main headlight switch and combination switch before the N7 relay. So these fuses are not live unless the headlight switch is in the on position and the turn stalk is relaxed or pulled to the flash position.

Check the following:
- Under the dash, look for the big connector that comes off the turn signal. Only large connector coming out of the column. This is a 14 or 16 pin IIRC. There are a couple smaller cables/connectors for the air bag, cruise switch, and possibly a steering angle sensor. Separate the connector.
Check across pins 1 and 3 on the side going toward the switch (pin side), should have continuity when the turn signal is at rest (neither pushed or pulled). Push away (hi beam) no continuity. Pull towards (flash) has continuity.

Now with the headlights in the "on" position, carefully jump across sockets 1 and 3 in this connector with a jumper wire. The headlights should come on if the switch is a problem. You may get a spark when you make the circuit, halogen incandescent bulbs are practically a short circuit until they heat up and make light.

Put the switch connector back together.

If that all checks out then check your N7.

Open the fuse box lid and then remove the 6 screws that hold the top cover on the fuse box over the relays. N7 is the long relay (about the size of a Snickers bar). (Hangry yet?)

Carefully pull the relay out (yes it just pulls straight up, with 34 pins it is in there TIGHT) and flip it over. There are three arrays of terminals:
Group X1 is the 16 pin array
Group X2 is the 10 pin array
Group X3 is the 8 pin array

The left low beam headlight is carried between X1:pin 1 and X3:pin 1 and should have continuity. This is the measuring circuit through the device. Basically a very low, but identifiable resistance value. When the light is on a very small voltage is developed across this resistor. The analog circuits in the N7 measure this voltage to verify current flow to the bulb.

Right headlight low beam is X1:4 to X3:3.

If you want you can also jumper across these two sets of terminals to see if the light comes on.

If the N7 is the problem you can try to fix it but it may be easier to find another one in a wreck. I think they kept this part constant across all the 124s and later (1986-up) 126s.

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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

When I had a very similar problem it ended up being the positive connector in the moulded bulb plugs. Both went out within a very short time of each other and I only figured that out after going through all the other motions of R&R and testing. Seems like a crazy coincidence but that's what it was!

Cut out the originals and replaced with some generic aftermarket and never had any problem since.

Might be worth it to remove the headlight bucket's back cover and pull and prod the wiring leading into the connector as a quick check.

Thanks Jay for the thorough diagnostic procedure guideline, I will check these and see what is found.

@Buster-Benz, I will check the molded plugs and see. I've seen the wiring melted before.

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k