I'm in the exact same position and I have a very odd problem. If I'm listening to the radio with Google maps on my phone and the voice navigation plays through the stereo (as a bluetooth phone call). It triggers the music or podcast player on my iphone and it starts playing. Even though I'm not listening to the phone via bluetooth, it still plays. This is most annoying when it starts playing a podcast I was in the middle of listening to.
I've searched EVERYWHERE for an answer and because it's so hard to explain, it's hard to find an answer