[Buehlar]

I don't believe there ever was an option to disable notification in question under "Sounds and Notifications". There must be a regedit for it, however, I couldn't find one.

Do you still get the pop-up with the radio off while you're not running Garmin?
If so, find out what is trying to establish a data connection and turn it off also when you use Garmin.

With that being said, I've never used garmin, does it require a data connection to download maps/directions like Google maps does or is all the navigation info preloaded on your device?