Hey I had been going through similar issues and tried all the usual suspects and for the life of me could not find the leak. My car is right hand drive so may be different experience to yours. However, What I ended up figuring out was that under the rain cowl where fresh air goes into the air vent which is protected by the plastic mesh, there is a sort of turtle-shell that shuts when you have recirculation selected on your aircon. When you turn your car off with just normal air-con on the turtle shell opens to pull fresh air in. The problem is if you park your car outside and live in a wet area then water can come through the air vent, passed the blower motor and into the passenger well.
Now I just have to make sure I have recirculation selected before turning the car off to ensure the shell is down.
Has anyone else had this issue?