Leaking heater core – See the inside of the windshield, just above the dashboard vent. An oily mist on the glass indicates that antifreeze has begun to leak from the core inner tube and is being driven upward through the vent. Then, check the floor in front passenger seat for an oily residue. The heater core is located in the heating ventilation and air conditioning (HVAC) unit, located just below the dashboard.
Make sure the humidity is not a result of a leaking windshield or open windows by searching for oiliness and color. Antifreeze in the leaking heater core may be blue, green or orange. Pop car’s hood and locate the heater hoses. Make sure the hoses are no gaps, cracks or worn which can indicate a leak. Also check that the hoses are foaming or oily residues. Next, start your car and turn on the heater and defroster.
Leave your car now normal running temperature. Gently touching both heater hoses. Both should feel relatively warm. If one is hot and cold, or if both are cold, this indicates that the flow heater core is blocked. Last, take your car to a professional auto body shop if you determine that the leaking heater core or clogged. Unless you have experience with Wake core adjustment or replacement, do not attempt this repair yourself.