If it is the VW plenum chamber drain problem you may not find it easily, you have to push down on the carpets quite hard as there's a lot of foam under there. Push down and it'll feel damp.
You need to make sure the drains are clear too, changing the filter on its own won't be enough.
Don't leave this too long as the alarm/convenience unit is down on that side of the car too, at least it is on Passats/A4s/Octavias, I think Golfs are the same. You don't want that rusting, although it's supposed to be in a waterproof box on new ones.
Incidentally I bought a bottle of Tesco screenwash recently and it absolutely stank in use, truly revolting!
SSE