Well I thought I'd take the plunge today and change the 3rd brake light as mine stopped working. Found a handy guide on here which tell you how to smash the red part of the light and take out the LED strip. Managed to do it all by myself :) and now have a working light again. The old one though was completely wet and mouldy inside so I think water from the rear jets had been getting into it.
Took about an hour but could defo do it in less time if I had to do it again. Feel quite pleased with myself saving a few hundred quid by not taking it to VW :)