if the unit is so well sealed, surely isnt it just excessive heat sweat from the roof spoiler as it does get quite hot and surely isnt there a depression in air presure whilst driving in certain weather conditions??
But it isn't adequately sealed. And on silver cars, they really get more than luke warm, even when parked in direct sunlight.
And we certainly don't get any kind of "formula 1" kind of aero effect from that tiny lip to cause any kind of depression!
I cant see how a washer jet is causing this? we are talking tiny droplets that cant escape, not a fishbowl effect! 
The washer jets are being squirted straight at the lamp. Try it for yourself. Infact, put your hand in front of the squirter, and get someone else to work the washer lever. There is quite some force there. The same force which "pops up" the headlamp washers, and they have a serious spring to overcome!
And it was a fish bowl. You could rock the car from side to side, and the water would literally slosh from end to end in the light!
Im sure its just like the Xenon headlamp symtom but the rearbrake light has no way to breath unlike the xenon has. 
Nope. Xenons generate considerably more heat than those tiny LEDs, and it is the change in temperature from the Xenon "bulb" which causes condensation inside headlamps.
