What is the cause of 1998 Renault megane water leak in driver's footwell?

Hi there in most cars not just in meganes if the leak is inside the car in either the drivers side or passenger footwell its normally the heater matrix which is located behind the dash board. But check under your carpets first as you may have a hole in the floor which would let in in rain water wen driving.

Try to remove the covers that are just below the front window (those with holes in them). They are easy to just snap off. Chance is big that you will find this area filled with water. This is due to a drainage hole that tends to get clogged on meganes.