That's quite common situation. Under the bonnet are to holes (one is located under the battery) allowing water from engine bay to go out. When some dirt or leafs stuck there, water level goes up and water starts going through air vents to the car. Have it in 2 passats. Easy to fix with just a screwdriver. You must make those two holes open again.
under floor in rear passenger footwell
Close to the rear axle, behind the rear passenger footwell, under the car.
If it is water, it could be a plugged up AC condensation drain or if it is engine coolant, it could be a leaky heater core.
it may be caused by the drain hole from the window screen being blocked it is located under the battery also check that the rain bar on top of the pollen filter is still there and not been left off allowing rain to travel in and down the heater pipes.
how to remove 2013 passat rear bumper
Removing the Rear Bumper of a Passat...
Is this the fuse box inside the rh passenger footwell. I have diagrams for a ML350 (my2009)
Under the hood, on the far passenger side in rear of engine compartment in plastic air intake plenum.
The 2001 Volkswagen Passat has 37.8 in. of rear head room.
The 2008 Volkswagen Passat has 37.8 in. of rear head room.
The 2012 Volkswagen Passat has 37.8 in. of rear head room.
The 2003 Volkswagen Passat has 37.8 in. of rear head room.