flsupraguy

I dont want to mount outside the car. I was thinking on glass by rear view mirror or on top of roll cage but worried about interference...

ProCoach

I have a lot of 991 GT3’s mount the antenna inside on the hard plastic of the dash as far forward as possible. While that is not recommended, no metal ground plane underneath and not 180 degrees of horizon seen, it still seems to work in several dozen cars that I have experience with.

Don't place it under carbon fiber or too near a radio antenna, either.