Leaking sliding door Trident 2009

Yes - it's a very well known problem with the factory sliding windows on the VW T5.

Our 2013 T5.1's window leaked from pretty much the day it was delivered, and out 2005 Trooper had leaked so badly (before we owned it) that we had to have the factory window cut out and replaced with an aftermarket replacement slider - which has been leak-free ever since (4 years, roughly).

In my experience it helps a lot to keep the seals clean. The VW window design may ~look~ elegant, but it is fundamentally flawed. The horizontal seal at the top cannot resist the road grit that inevitably gets deposited on it while driving in the rain, and that grit gradually works its way down to allow water to leak through. But they are a bit fiddly to clean, I must admit!

The window on our 2005 van had been leaking for quite a while before we bought it, and the steel understructure of the lower guide rail had rusted and distorted, so that the window was no longer pressed properly against the fixed glass. It was not repairable (because the lower rail is bonded to the glass) so we had to have it replaced. If you never plan to use it as an opening window then a cheap and effective solution may simply be to seal it closed.