Water ingress to our cars is frustrating at best but can be expensive if not dealt with.
The plenum is a favorite and swiftly followed by a sunroof if you have one.
Then the AC drains also need looking at but less frequently.
Have a look at this
video for doing the AC ones but by a set of clic r pliers from
ebay first so you don't wreck the hose clips.
You should peel up the carpets also and make sure they are not saturated and sitting on top of a puddle of water around the phone module etc.
Also have a look through the common faults in D3 section for the drain threads, here's mine
http://forum.a8parts.co.uk/showthread.php?t=9680