hi, try looking under the bonnet at the gulley underneath the brake cylinder and heater cover, if there is water in it, the drain holes under the heater cover where the motor is are blocked which means the water fills up and gets into the car through the heater ducts under the cover.
the heater cover is in the middle at the bulk head, it has vented cover over it, just take a torch and look underneath it for water.
Had the same problem on mine, took 3-4 attempts to clear the hole because you can't see them.
hope this helps.
Mick