I have a similar problem with a puddle of water in the rear seatwell.
I have not fixed it, but I am almost 100% certain that the water is coming in through the heating system (there is a air heat duct under the seat just next to the puddle).
The first time, I thought it was due to drains below the windscreen being blocked and water backing up and into the air intake.
However, the problem returned even after clearing these drains.
Other possible factors: being parked on a slope; with rear wheels lower than front whels; allowing snow to sit on the windscreen and then melt.
??
Has anyone here solved this?
Thomas
February 2009