Not sure if others have found the same solution, but I think that I have a permanent fix. I had one tail light on the liftgate that was filled with water after a car wash, and dirty applique lenses as the video above showed. The seal/gasket on the back of the lamp was in proper place (not dropped down as many have shown), but it no longer made a water tight seal with the body panel (water went in from behind the gasket).
My fix was removing the factory seal/gasket and replace it with butyl seal (I used 3M Windo-Seal). First try, I used one bead of the butyl strip, but that didn’t seal completely, so I doubled up, putting one strip on top of the another. To test, I pressure washed the area and did mot see any moisture in the lamps (I also did the other lamp that hadn’t show problems yet but I felt I would eventually. In fact, I also did the applique).
To remove the lamps, I removed all the plastic trims on the liftgate (top, side, and bottom, and unplugged all the wires). I removed the applique completely, because I wanted the lamps to go back in straight instead of having to slide them in from the side if the applique is place. I was glad I did because after removing it, I found that the seal in the upper edge of the applique had failed and allowed water to drench the camera and the license plate lamps from the top (there were already moderate amount of corrosion). I replaced that seal with one layer of butyl seal.
Removing the applique (I might be misspelling this BTW) was tricky. I removed it along with the plastic housing that holds the camera and the plate lamps as one piece, instead of prying it off. To reattach the applique I used one layer of butyl seal along the upper edge, and to the lamps, I used 3M auto moulding tape.
I have used the same butyl seal (for taillights) and moulding tape (for plastic body kits) on other cars before and they are still fine after 20 years in Southern Ontario and Northern US weather so I am expecting it to perform equally well in the P85+.
I didn’t take many pictures during the process.
Hope that this is helpful.