Compare the Navigation settings from the Android and iOS versions of Google Maps.
I wish Google would add the option of "Play as Bluetooth phone call".
If I want to have the navigation through my VW Golf (64 plate) speakers, I need to put the car stereo into Media mode and on Bluetooth, then drive in silence unless I stream music or radio from my phone. At least with my work iPhone, I can have the radio or an SD Card on and the music stops for the *navigation* phone call, then continues with whatever was playin after directions finish.
{edit}Screenshots from a Huawei Honor 7 (Android 6.0, Maps 9.25.1) and an iPhone 5S (iOS 9.3.1, Maps 4.18.0){/edit}

