I had the same with the drivers side floor full of water and traced it to the seal on the brake pedal mounting plate
Removed the master cylinder and servo, clean out the seal which had gone brittle then using mastic sealed the whole area, also the bulk head drain flap was jammed and full of leaves. put it all back together and water tested no problems - 6 months since still dry
Mark
September 2011
its most likley to be the heater matrix
marc
August 2011
check roof aerial is sealed at base,check drains at bulkhead aren`t blocked
cdc
August 2011
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