ok its not a crack. under your carpet is an inch thick piece of styrofoam with sound deadening on one side. unless its punched through that, then gone UP to the carpet, then its awesome water that can run uphill.
im going with andys theory of aircon condenser is leaking or its possible the external vent behind the battery is leaking into the cabin space around the aircon unit. or through it!
youre going to need to remove some panels and see what is damp