But how do you know its that water thats getting in?? i cant see how that can be it, unless its actually squirting directly at the light house
then making contact with the glass 1st. There isnt enough pressure surely 
But thinking about it, every single Mk5 I have ever driven/looked at has had the rear squirter aimed straight upwards, in the 12 o'clock position - ie
| - which then drenches the 3rd brake light. However, in the official Volkswagen AG workshop manual for the Golf 5, it clearly shows that the squirter should be aimed away from the brake light, in a "2 o'clock" position - ie
/ (actually leaning over a little more to the right) - which then clears the rear light, and the light stays dry!
So therefore, if the high level lights were designed so that the rear squirter was not aimed directly at it, but the factory or the stealers now have them aiming straight at them, then they obviously arn't gonna cope, and the spray will overcome the seal and fill them up.
The water in mine is most definately clear, with a hint of froth identical from the VW screen wash additive. I wasn't aware that rain defied the laws of gravity

- and it isn't a murky muddy water from what generally collects on the rear screen.