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I had this.... turned out to be the rear light gasket was letting water through. The water travels under the boot carpet, under the back seat and down to the front of the car. If yours is the same, i'd get it sorted as enough water staying in one place aint good for ford metal
drivers side is not to bad its the passanger side that is bad and its only just happened with this really bad rain in the last few weeks plus the under dash carpet is wet
had this with my last rs after a new battery tray it turned out to be the seam between the inner wing and the bulkhead just infront of the battery tray . water was seeping through once i sealed it up never happened again