Hello,
This issue is likely caused by inoperative A/C compressor.
When the defroster gets turned on the A/C kicks in to dehumidify the air, this helps to evaporate moisture from the windshield.
https://www.2carpros.com/articles/car-air-conditioner-not-working-or-is-weak
So, check to see if the A/C is coming on. Sometimes the A/C compressor will not turn on due to being low on Freon. It can also have a bad relay for the clutch.
The relay is located under the hood (see diagram).
Check the relay for any corrosion or burnt pins.
Here is a procedure on how to test the relay:
https://www.2carpros.com/articles/how-to-check-an-electrical-relay-and-wiring-control-circuit
I have attached diagrams for your reference.
Please let me know if you have any questions.
Thank you.
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Wednesday, October 27th, 2021 AT 11:38 AM