This might be common knowledge, but check your cowl/scuttle drains if you have an older car!
A couple of weeks ago we had 50mm of rain over a short time, an the passenger side footwell was full of water and wet up under the dash on my Peugeot 407. C5 x7, C6 and 508 mk1 are very similar but i guess it applies to most cars..
I couldnt find any leaks from the windscreen when pouring water down it. It has been done a few months ago, but by my collauge who is careful and doesn´t get any comebacks.
But when i removed the plastic cowl i could see there was lots of old leaves, pine needles etc under the wiper motors (which also isn´t good as they often fail due to moisture getting in).
Removed the covers over the strut tops to remove the wiper motors, and i could probably ave filled a 5L bottle with the debris. I also vacuumed up some and blew out some with compressed air.
My guess this far is that it overfilled the scuttle with rainwater, so much that it got in via the cabin air inlet.
