Where is the water coming in from

There are a few possibilitities, the first if it is is happenging when the heater is on;

The heater uses a mini-radiator called a heater core. There are 2 hoses that run back and forth to the block/head that are controlled by a valve that is controlled by the heater temperature contorl. The more that it is turned up, the more flows through it. Eventuallly, it will leak bad enough to collect into heater box and run onto floor. You can by=pass this for a while by running one of the hoses from the engine back into the other engine connector.

The second thing that can happen does not involve the heater core and will not have fluid in the floorboard or coming through vents that smells like anti-freeze. What happens is that there is a fresh air intake box or cowl just infront of the windshield. Depending on the care, some are removable externally, others have to come out under the dash. After time, build up such as dust, leaves and debris bill up the box and cover up the, "weep holes" which allow overflow to drain into the fendrwell just behind the rear tire. Soemtimes the airbox can rust out and cause the same issues. That a blocked one can. Usually they are set up to handle the flow of water but after time the blockage gets so bad that it bakcsk up into the fresh air intet which is above the A/C Evaporatetor to the dight of the heater box. It willl take some time to get bad enough to fill up and start making a puddle in the car passenger side floorboard. The only thing you can do is to remove the blockage and/or fix the rust and clean the box out. It will be the worst when the fresh/re-circulate is set on freash air. Theis opens the flap that the water spills over and into the interior. Another thing that can happen is that the plugs that hold the peices of the cowl in place ca leak over time and need silicone sealant.

If the heater core is leaking I roccomend replacing the thermostat, any damaged or worn hoses or clamps, clips, seals, heater core valve and possibly cable as well as the core itself. Also, flush out the sytem and use silicone whereever it appears that a leak might occur.

If the box is leaking, replace all the plugs and clips that hold the fresh air box in place and well as any componenents of the cowl that are made of plastic. The windhsield gasket is also a good ides to replace in both istuations unkless it is a rather new car and it is a, "Glued" in type windshiled. Again, place silicone anywahwere that it appears anecassary and replace any seals or pasctiv componenets in cowl intake.