Leaking Heater Core
A leaking heater core or a clogged drip tube results in puddling on the passenger's compartment floorboard. The heater core is in the air distribution case. It's part of the cabin temperature and the engine's cooling system. The heater core transfers heat from the engine's coolant to the passenger's compartment. Depending on the system, this flow may be controlled with a heater control valve or flow continually whenever the engine is running.
If a leak occurs in the heater core, the coolant leaks onto the vehicle's front floorboard. It may also drip out of the vehicle's evaporator drip tube and be seen on the shop floor. Leaking coolant results in a sweet smell, especially in defrost. It can also fog the windshield. Check the vehicle's coolant reservoir; if it's low, pressure test and inspect the heater core for leaks.
The drain tube at the base of the air distribution case can become clogged, preventing moisture from draining. Instead of draining outside the vehicle, it collects in the case and drips onto the floorboard. This clog results in a wet carpet on the passenger's side of the vehicle. Sometimes, they can be unclogged with an evaporator cleaning kit and a piece of flexible wire. Do not poke a hole into or damage the fragile evaporator core.