I personally use both the Rain-X repellent and the Anti-Fog application. They both work wonders! Just be sure to follow the instructions as directed, sometimes you may need a bit more liquid depending on how foggy your windows become in the winter.
I recommend cleaning both the interior and exterior of the windshield before applying.
You can also apply to the side and rear window, just keep in mind of the defroster heating lines as to not damage them.
Use at your own risk in that instance!
I hope this helps!
Sincerely,
SC