Listed below are 4 probable sources for the water problem.

1. Problem:  Water drips from the end of the cowl lacing fastener clips onto front floor carpet . ( The cowl lacing is mounted on the engine side of the fire at rear edge of the hood. This runs the length of the fire wall, sticks out about a 1/4 " at the top and is held down by the cowl sheet metal.

  Analysis:  The cowl lacing fastening clips if not properly sealed will allow water to drip directly through the heater core into the bottom of the housing and accumulate to a point where it will run out of the damper pivot and unto the floor .  This problem is sometimes misdiagnosed as a windshield or cowl plenum leak.

  Solution:  Seal the lacing clips and test for a leak.

2.  Problem: The windshield gasket may be leaking , allowing water to soak through the windshield rubber gasket between the heater core and the plenum chamber and unto the floor .

Analysis:  Water is leaking past the windshield gasket at the heater core housing .

Solution:  Remove the heater core housing and check the windshield gasket for moisture. A windshield leak may soak and saturate the gasket between the heater core and the plenum chamber and run into the housing or directly to the outside of the housing and unto the floor . If the windshield gasket is saturated with water seal it by applying sealer on the inside behind the rubber lip, to outer edge of the pinch weld flange along the top sides. While the heater core housing is out seal the seams around the baffle in the plenum chamber by reaching in though the opening in the dash and toe board assembly.   When finished test for leaks before final assembly.
 

3. Problem: The plenum chamber baffle may be admitting water through voids in the sealer around edges of the baffle .

Analysis:  Gaps in the sealer allows water to pass the baffle edges .

Solution:  Clean off loose and dried sealer and fill all gaps with fresh sealer .  Test for leaks .

4. Problem:  There may be some spillover at the flange around the heater opening when car is parked at an extreme attitude .

Analysis:  The plenum chamber drainage system is capable of diverting water at the rate of 10 gallons per minute without spillover at the flange.   Under extremely unusual parking altitudes or with a heavy windswept rainfall water may spill over the flange .  The water will then run down through the heater core into the housing and out onto the floor .

Solution: Install a rubber gasket on the flange which will raise the effective height of the flange at least one inch . AMC had a kit for this purpose ( part #3598667, Group 20.120 ). The deflector in the kit can be installed and sealed by reaching through the heater opening on the underside of the instrument panel.