Take the scuttle panel off and clean the drain holes in the corners, common fault on lots of cars, drain hole blocks and water comes in though the heaters, had this on my old mk2, i found out the hard way that they were drains after i cleaned the leaves then filled with silicon, oops. aslo had the same problem on a focus and have heard about it on other cars!
The drain hole is normally full of leaves and stuff like that.
Hope this helps!