Sounds (smells?) like a leaking heater matrix. Check your coolant levels.
For some reason, I read it as 'back passenger footwell) and was trying to work out how water could get back there. Yes check Aircon condenser drain and heater matrix.
If the leak is 'large' and from the heater matrix, the level in the coolant reservoir must have dropped noticeably. Hopefully you look at it occasionally, and know where it usually is ?
