renault clio 1999 water intake behind driver seat?

After heavy rain there is a considerable amount of water in the footwell behind my drivers seat. It appears to be building from underneath and rising up into the carpet. The door seals are fine. I can hear the water moving across the car when I turn corners. In the last few days I can now hear water trickling when the heater is turned on but only on the number 3 and 4 settings. Any ideas where it's getting in and collecting at?