The inside of the windshield can turn greasy rather than merely foggy because a persistent interior haze can come from cabin materials, smoke, cleaning products or, in some cases, a heater-core leak. The pattern matters more than wiping the glass again.
Film from the cabin
Interior glass haze can be produced by volatile compounds from plastics, smoke and detailing products. That film is not ordinary fog.
When a heater core is involved
Coolant leaking from a heater core typically brings extra clues: coolant loss, a sweet odor or persistent misting. Those clues separate it from a film that comes from plastics, smoke or cleaning products.
