My tailgate LED stop lamp stopped working - sometimes.
I've already taken off the spoiler once to check wiring, LED lamp etc. Everything fine, even when back on the car again. Now it's dead.
Before you eventually try this procedure, double-check if the wiring is ok. It is easily done with a multimeter:
* Take off the spoiler (explained by many others)
* Disconnect the LED connector.
* Check both pins in the plug coming through the flexible rubber conduit.
* They should show close to 0 ohm vs something you definitely know is ground on your car.
* When measuring, move the wiring back and forth to check it is not broken somewhere inside the conduit.
* You could also check the LED lamp itself by applying 12V (external battery of some kind) to its connector.
* The unit does not die if you connect with wrong polarity. It's protected with diodes. Try both ways.

Being curious by nature, I decided to give it shot and disassemble the thing, although having read elsewhere it's not doable.
Yes, it is!
But not with a hair dryer or heat gun. The right tool seems to be something like this butane driven solder iron equipped with a melt knife tip:



Carefully, melt a small groove in the slot between the glass and the base. Keep the front part (the one pointing backwards on the car) untouched:



You might need to "help it a bit" by carefully bending the glass and base apart while melting.
With a bit of luck - like me, you get the two parts separated in re-usable condition:



Now, it is easy to get lenses and LED strips out of the base
(one of my lenses was broken in two, but that does not really matter too much):





My LED unit was not working because of a broken soldering of the short wire between the two LED strips:


I freshened up all solder joints on the strips just to be sure.

After testing that the LED's were doing their job, it was time for assembly. I glued the two pieces together temporarily with super glue all around, and kept them tightly assembled for a few minutes with tape:



Then, I removed the tape and filled the slot between the glass and base completely with hot glue (or the favourite sealant of your choice) + a thin stripe at the front (the one pointing backwards on your car). Filling the slot does not interfere when mounting the LED lamp back on to the spoiler.



LED lamp back on the spoiler. Spoiler back on the car. Job done. The whole procedure took me approx 2hrs, and I saved £90 'ish for a new XFG000071.
Actually, if your LED unit is completely dead, it should be possible to use any 12V bright LED strip if it fits into the base unit. It does not necessarily have to be red LED's. White gives more light, as it will be colour filtered only once - through the red brake light glass.
Happy braking!