Water entering at the cowl intake vent just below the windshield can let some water enter the heater matrix--there's a drain tube at the bottom. The rest of the air/water spills to each side and into the rocker panel chamber. There are large access plugs to the rocker panels--remove the step plates and fold the carpet back to expose them. The drains along the bottom of the rocker panels must be clear. You don't indicate if your car has a factory sunroof--if it does there are drains at each corner-the rears going through the trunk. I always look first at the seam where the inner rocker panel meets the floorpan. A rust hole in the middle of the pan can be easily fixed (often caused by leaky windshields)--the edges, not so easy. -- Dave