If your vehicle is showing defroster is not clearing the windshield, here are the most likely causes and the parts you'll need to fix each one.
A severely dirty cabin air filter restricts airflow to the windshield. If recently replaced, it may have been installed backwards, blocking the blower's intake.
The blower motor pushes air through the vents. If it or its resistor fails, you will have weak or no airflow coming from the defroster vents.
This small motor directs air to the windshield or blends hot and cold air. If it fails, air might only blow at your feet or face instead of the glass.
The defroster relies on hot engine coolant to heat the air. If the coolant is low or the thermostat is stuck open, the air won't get warm enough to evaporate moisture on the glass.
The defroster automatically runs the A/C compressor to pull moisture out of the air. If the A/C is broken, it will blow damp air, making fogging worse.
A leaking heater core will spray sweet-smelling coolant vapor directly onto the windshield, creating a greasy, foggy film that the defroster cannot clear.