W126 blinkers missing, dash lights missing
 

I took my 87 300SDL in for a headlight change and caliper change.

Before hand, the left turn signal worked, but the green dashboard arrow didn't blink & there was no clicking-on/clicking-off sound in the cabin.

After the repair, the dashboard is dead and I have no left or right turn signals. None of the lights on the dashboard work. For example my low washer fluid reservoir indicator is not on, even though my reservoir is low.

It was closing time at the shop. At the time, all I noticed was that the turn signals weren't working. I hadn't immediately noticed the whole dashboard was dead. They poked around the fuse bay, rejiggered fuses & relays, & checked some electrical connections there, and didn't turn anything up.

As I was driving around, I moved the dash light rheostat. At first the light over the LCD adjusted, but then the light went out.

The car's odometer has been dead for ~6000 miles too (approximated via fuel logs).

Anyone have any ideas about where problems might lie? What things would you look into?

Coincidental issues.

The rheostat in the dash has likely died. Replacing it or bypassing the terminals will get you lights back. Since it blipped over the LCD when you fiddled with it, that's proof enough that it's trying to work.

Are your fuses aluminum or copper/brass? If they're aluminum, time for a system-wide delete and replace. Seriously, it just isn't worth the headache to try troubleshooting with those POSes in place.

You may have ground issues. Behind the cluster is a large group of brown wires bolted to a stud or two. This is a main ground point and is a common one to go bad. It causes all sorts of strange issues. Unbolt, scrub all connections with a wire brush, smear on some dielectric grease, rebolt.

While the cluster is out, pull the speedo and install a new set of odometer gears. Search this forum or the web, there are tons of how-to's out there.

Turn signal problems may be the turn signal relay, the hazard switch in the dash, or the combination switch. Turn the hazards on. If they all flash you know the relay and possibly the hazard switch is good.

These cars have a light control module so you can be notified when you have a bulb that's out. All of the exterior lighting in the car passes through it and sometimes these can BE BROKEN and cause lights to work intermittently or not at all.
https://lh3.googleusercontent.com/iw...Q=w800-h453-no

Broken solder joints and poor connections are not "shorting out". Shorts cause blown fuses, melted wires, and fires. Broken solder joints are open circuits, things just stop working.

Fix your odometer while you're at it
 

You drive a Mercedes Benz. Why drive it around with stuff not working? Get it all fixed and have a nice car.

I edited my post just for you.

On one I observed the parts inside had melted like a fuse.
https://lh3.googleusercontent.com/Vb...8=w800-h453-no

This may not even be the problem, but I thought it would be helpful for you to know about it because I was chasing down lighting problems all over and then it ended up being this bizarre part that I didn't know existed that was causing all the issues.