drivers footwell leak?

Hi,
You don't say which car but if the leak is not external to internal as you suggest then the obvious area to look is at the heater system within the cabin. Check if you are losing water from the reservoir and if you are check the heat exchanger under the dash or behind the central dash area.
Cheers now

Oldgrubber
June 2019

check that the membrane between drivers door and door trim panel is fully intact and fully stuck to door at the edges,

k
May 2019