WHY DONT BOTH OF MY HEADLIGHTS WORK AT THE SAME TIME?!?!?!?!
 

EDIT:( Working on an 87 300D U.S.)
I have replaced both bulbs checked and replaced both fuses and checked all the connections. some times my right headlamp doesn't work and then sometimes my left headlamp doesn't work.
They both work some times and sometimes only one works,
(BOTH HIGH BEAMS WORK)
It is driving me crazy,
I am at a dead end.
Are there any relays for the head lights I don't know about
Any help would be greatly appreciated.
Thank you in advance for any help you can give me

I've had this happen, for me it turned out to be a bad ground in the headlight wiring. One would also be extremely dim while the other was one and vice versa.

I took a light tester to the light that works right now, found which prong should be getting power, went to the light that isn't working right now and found NO power going to it. ???????????
I also checked the fuse block and both fuses for the low beam lights
(#13 & #14) They are getting power at both sides of the fuse.

If Im not getting power to the light with a tester, does that still seem like a ground issue, since the tester had its own ground?

Year make & model?

My bad. its a 1987 300D turbo

Now Neither of them work!!!!!!!! ARRRRRRRRRRRRRG!!!!!!!!

I am about to remove the low beam connector out of the headlight wiring socket and insulate the end of it.

Then run a new wire off the top of each designated fuse to the headlight sockets.
The fuse is dead if the lights are off, so this should work for now.
I will also check my ground wires but beings that the high beams work fine and I think they share a common ground I dont think the ground is the issue here, But I am not ruling it or anything else out for that matter right now.

Does some one know where I can find a headlamp wiring diagram?
w124 87 300D

Thanks again to all that can help
This is my daily driver and for now I cant drive it at night. or in heavy rain

Am I running the risk of melting things down by doing what I described above?
Are the headlight relays placed before or after the fuse block?
If they are before the block then i should be fine.

I found the problem. I think, the black Hella BOX under the cover behind the fuse pannel. it has the part number 126 543 01 32.(sounds like a W126 carry over part) there are 4 of them look like a sheet metal Maze's that run electrical current through them. one of mine look burnt up & diminished in spots. i carefully bent the metel so it was touching again, plugged the box back in and the headlights work now. so I will be replacing that part this weekend when I can get to a local PnP.

EPC startek has the diagrams. There's a link somewhere.

The lamp failure unit... ironically causing the lamps to fail.

... this may solve the issue I am having with my 87 TD... thanks!

I did alot of research last night on this problem, It seems that a TON of people are having this problem, I noticed most people have done what I did by replacing fuse bulbs checking connections etc...
most fixed the problem in the end by replacing the above mentioned BOX
one guy said he replaced his colum mounted dimmer/multi function switch
when that didnt work he replaced the malfunction box, when that didnt work he thought about replacing his headlight switch but instead he just grabbed the wiring harness behind the switch and gave it a good tug and everything worked fine. It seems like Mercedes has Sh#t ton of stuff working against leting the headlights work.

Hope you get yours figured out.
BTW have you got your car back from the trans. shop yet?