Just had my 2012 ML350 at the dealer to check another issue, and they told me that both of my tail lights (only the part on the trunk door) were faulty and had to be replaced.
They didn't check in detail but gave me a price of USD 1300 to replace both. Not sure if they really checked at all or if they just refered to an error code. This is roughly 5x the price of what I can buy replacement lights for myself online.

Before ordering new lights, which is still approx USD 300, it just occured to me that it sounded a bit strange that both tail lights on the trunk door would fail at the same time. I really dont want to order new lights only to find that the issue is somewhere else.

Any good ideas for what I could ckeck before ordering new lights?
Does anyone know of any fix that would be cheaper than simply replacing the lights?

if i were you i'd seek a 2nd opinion on it, if they say the same thing, buy the assembly parts (or get it from people are parting out their ML's) and just pay the labor, which will be less than 1300.

I wrote the above questions while waiting to pick the car up.
After looking for myself I see that both lights are indeed faulty. The lower parts of both lights does not work properly. The lowermost lightstrip on the left side does not work, and the two lowermost lightstrips on the right side does not work.

Is there any way of repairing these (myself), or do I have to order new light assemblies? I'm thinking in the line of fixing some bad wiring/soldering points or changing out some easily available LED-chips?

it doesnt work like that because if you examine both left and right assemblies closely you will see that they are all LED strips on the assembly itself and you cant replace each individual strip. Your best bet is to buy the whole assembly, again either new or used (from someone parting out their ML). I wouldnt advise to get it via dealer though

I am having the same problem..The left one ,bottom strip doesn't work and the right not working at all ..I plugged the right one (after altering the coupling) into the left socket and it works!!! ..now ,I am thinking there's a fuse problem ,but CAN'T FIND the damn fuse ..Checked the rear seat box,passenger side and under the hood and took them all out with no luck...Does anyone knows where is the fuse for the trunk taillights? Also ,do you guys get water in these ,it looks to me that there's a common problem after looking at other cars and I think that is the issue that they fail.

I think I might have seen some condensation inside the lights in the past. These are LED, so they should normally not break unless they are exposted to water or high electric currents.
Was it easy to remove the light assemblies? Could you please wirte a short how-to?

I'm thinking of checking the internals of the light to see if I can spot some obvious corrosion, bad soldering, loose wiring etc.
I would also like to measure the current from the LED drivers just to have an idea if the problem is with the driver or with the LEDs themselves. I have seen some pretty simple and cheap fixes of BMW LED-lights. None of these are made to be repaired, but that does not mean that it is not possible or even hard to do it. It makes sense that the lower strips would fail due to water. If that is the case my bet would be either bad contact due to corrosion or a shorted LED driver. A 12v LED driver should be easy to find on ebay for a few bucks, just need to verify the output current (3.5V?) My biggest concern is how it can be opened to check the internals. I fear that it could be glued together...

I took them out twice and there is only one tab with a screw in it ...bad design, the left one was already broken (bought the car used) and there was water going into the trunk .
You will have to take the inside trunk cover off -pull and it will come off,then you will see the taillight screw ,unscrew it , wiggle and pull towards the middle of the car. As for taking them apart ,I wouldn't recommend,they were molded together and unless you'll cut around ,I dont' see any other way of doing it. Inside you'll find a board with circuits and electronic stuff,don't know too much about or how to describe it.
Make sure than when you'll put them back there is no opening as the water will get in ,use some automotive silicone around the seal,not visible and does the job.

It was working until I took it out to fix the water issue.I moved it all directions to get the water out ,plugged it back in ,was flickering and then dead ...plugged it in the other side -working.
I checked the wires and no current..very strange ,I know it's working ,but no power on the wires and wasn't able to find any fuse blown.
I am thinking that it shorted when I tried to clean the water and screwed up a relay or a module for the right side..Have no idea and any input would be appreciated.