Alcatel idol 3 wont interrupt car radio with nav app directions When connected to car bluetooth, google maps or waze wont play on car while car radio is on. If device is selected on car unit then you can hear nav app turn by turn directions. But if radio is on, directions wont override radio.
Your car radio  is able to play sound depending on which input is selected. If it's radio then your car will play radio, if it's Bluetooth then your car will play sound from the paired Bluetooth device. 
Your radio cannot play 2 different input devices at the same time. 

Also when your phone is paired there exists two audio streaming options. Your phone can stream either media or call if your receiving Bluetooth device supports it. Everything that is not a phone call / Skype call / whatsapp call etc gets the media output.