I found a YouTube video that explains exactly why this is happening and it will happen to every Durango unless later years have a more durable seal/gasket. Per the video there is a thin almost paperish type of gasket on the inside of the entire expensive unit that dissinigrates over time and will eventually let water in. The video does a good but quick look of comparing a new gasket to the old one that is flaking apart. Insane that this is over a $1000 fix for a poor gasket/seal design that if not changed by Dodge would seem to happen to every Durango. It's just a matter of time...
These assemblies are far from being weather-tight when made, and no, the FCA engineers have not addressed on the 18's yet. If you removed it from the tailgate and threw it into a lake, it would eventually sink. The video's explanation of a "gasket seal" is also erroneous. The only weatherproofing provided, is to keep water out of the interior of the hatch itself, not the taillight assembly, (unfortunately).
I noticed the incandescent bulb holders on the video's original assembly weren't even opened. They could've drained most of the water out right there, on-camera --- not that it would've corrected the failure, though it actually DOES sometimes, as I've read somewhere here.
The best way we have to
prevent this failure, is routinely-inspect it for water pooling in the bottom. And if found, to remove the bottom retention bolts, and/or to drill a hole in the bottom left & right corners.
Of course, LEDs last a lot longer than incandescents do, but they will still fail eventually. They sure don't like water, so it's not fair that FCA failed to design this mega-buck array to be exposed to standing water.