Most water leaks in the passenger side of a car come from environmental water such as rain or from heating and air-conditioning systems. Excessive moisture leaking into the passenger compartment may raise humidity and cause electrical problems.
Leaking engine coolant may cause warm, sticky spots in the passenger compartment. Mechanics confirm this leak by checking to see if the coolant reservoir is low. Water leaks in the passenger side may also be a result of a faulty evaporator core. If the blower fan draws dust and debris into the evaporator, it may cause water to back up and drain into the passenger compartment rather than under the vehicle.