One possible cause is that the body drainage holes are blocked so that the rainwater that runs down the windscreen cannot drain away and eventually enters the heater air intake. Remove the debris blocking the drain holes in the scuttle below the windscreen and check that water drains freely. If the drains are blocked a friend has successfully used curtain wire to clear them.
This is a common problem on a lot of vehicles, irrespective of make or model. I have had it on my Scenic, my old Cavalier and on my wifes Saxo.
Another possible cause is that one of the rubber grommets on the engine compartment bulkhead has popped out and water is getting in that way. If you dont know what the grommets are, they are the rubber 'bungs' that the electric wires pass through.
Hope this helps
AcmeUK
March 2009