If it smells of antifreeze and your coolant level is going down then it will be your heater matrix.
If it is rainwater, (or melted snow), then check in the engine compartment in front of the bulkhead at each side. The drain holes may well be blocked with leaves etc. Clear them out and that should cure your problem.
Take the mats out and dry them thoroughly otherwise you'll get condensation in the car and your windows will mist up all the time.
Chicks Fan, February 2009