My defogger is not working?

Hello, I'm Danny.

It sounds like you might be having an issue with the defrost actuator on the HVAC box that is defective or failed. This actuator is a small electric motor that opens a door to direct air flow through the defroster vent. Here is a tutorial showing how to replace a blend door actuator which is similar:

https://www.2carpros.com/articles/replace-blend-door-motor

I've attached picture steps below on how to remove and replace the defroster actuator below and its location on your Impala . Hope this helps and thanks for using 2CarPros.