The third brake light is a poor design - surprising coming from MB.
The problem is that the arrangement of the assembly contributes to its own
destruction (something like the Third Reich).
If you look at the arrangement, you'll see that the spring bracket actually
hangs on to the threaded studs coming from the rear of the plastic light
assembly. Every time the trunk lid is slammed down, the inertia of the heavy
metal bracket tends to force the rear of the assembly down, eventually
causing it to fail.
I have devised what I think is a solution to this problem.
The lowest price I have found for the light assembly in the U.S. is from
Parts.com for $91.20 plus shipping. My MB dealer matched this price (and
ignored the shipping cost).
After removing the old assembly, be sure to clean the lip of the opening in
the trunk lid. This is where the adhesive gasket of the new light adheres to the lid. Use naptha (lighter fluid), Coleman fuel, or mineral spirits. You should note that the light fits properly only one way. On mine, the part number/writing on the back of the light went in upside down.
Next, remove the paper from the gasket and insert the light into the
opening. While holding the light assembly in from the outside of the trunk, apply
silicone RTV (I used clear, but any color will do) liberally around the
perimeter of the back of the light assembly inside the trunk. It helps to have
the use of someone's small fingers to force the stuff on to the top and
Assemble the metal bracket to the light with the two nuts. This ensures that the light is positioned properly in the trunk lid. Try not to get too much RTV on the bracket. Leave it this way for about 24 hours to allow the RTV to cure.
Now remove the nuts and carefully remove the bracket by pulling it
rearward. It will have been glued to the rear of the light, so you might need to use pliers to do this. Be careful not to rock the bracket up and down too much so as not to break the adhesion of the RTV to the trunk lid and the light.
Now apply more RTV around the perimeter.
Allow another 24 hours, then reassemble the trunk lid parts.
What you have done is glued the light assembly to the trunk lid. The RTV
acts as a glue, seal, and shock absorber. It is a somewhat permanent
solution - the LED's will likely outlast the life of the vehicle. If the light is
damaged, it can still be replaced beause the RTV remains soft and can easily
Any good RTV can be used. I used the type that comes in a cartridge that fits in a caulking gun. It is also available in squeeze tubes.
You might be tempted to leave the metal bracket bolted to the light assembly, but it really is not necessary because the light assembly is not very heavy and the RTV will hold it very securely.
Although this procedure sounds time consuming, it will likely preclude your ever having to buy another expensive third brake light.
If you experiment a few times, you'll find that it takes very little downward force to close and latch the trunk lid. This will also preclude damage to the light no matter how it was installed.