If the HVAC (heater) housing is not properly sealed at the bulkhead (firewall), water can leak into the passenger compartment. Foam sealant should be used to seal any gaps and the evaporator drain tube can be modified to prevent further water entry.
Car Problem Reports
Dodge Caravan Water Leak Due to HVAC Case Not Sealed at Bulkhead
Dodge Caravan Problem
Visitor, 2003 Dodge Caravan, 110,000 mi
Water on floorboard of passenger side noted. Pulled trim and carpet to find puddles and soaked mats. Cleaned evaporator tube for HVAC which may have staved water flow some when A/C running but otherwise, still leaks in some. Plan to put silicone or this "foam sealent" around tube at exterior and as best as one can on interior then add a L shaped tube to the evaporator tube to divert flow groundward rather than allow it to flow back to firewall (bulkhead).
Visitor, 2003 Dodge Caravan
Water from AC leaks onto passenger floor