either the wiper cowl/cabin intake drain points are blocked and flooding into the intake or most likely the rubber seals for the clips under the windscreen that holds the windscreen cowl has perished over the yrs and water leaks from windscreen through the broken seals and straight down the cabin intake fan onto the passenger floor
[ATTACH]15117[/ATTACH]
(as is what happened in my case during a monsoon)
(pic heavy)
http://www.micra.org.uk/threads/35251-PollyMobiles-Rebuild
also check the ecu hasn't been soaked by the water cos that could short it
