In my case the footwell was getting wet. Behind the glovebox was soaked and the car was extra steamy with the heating on. It was until I had water dripping on my shoe from the fuse box under the steering wheel I realised I had a problem. Under the plastic scuttle panel passenger side is a small drain for water to run down the wing and onto the floor. This is pretty narrow and gets blocked with crud. This causes a build up of water which when bad enough to overlfow into the heater vent and land in the blower motor. When you have the blower on to clear the windscreen or to keep your toesies warm, it blows all this water out through the pollen filter, this is where it drips down. An easy way to check is to reach behind the glove box and unclip the pollen filter clip at the bottom, pull it forward and you can just about get a digit or two in there. If the filter feels wet or you feel moisture, chances are your scuttle drain is blocked. Don't take this as Gospel, it's my 0.02 from my experiences.