Why does my audiobook quit playing when my phone screen fades off?

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I am using an iPhone 6 plus. I downloaded an audiobook last night and played in in my Ford using synch Bluetooth media stream. Last night I had no problems, but now the audio fades off every time my phone screen turns to black. The ford.com synch instructions did not address my issue or perhaps I couldn't find my specific problem. Does anyone have any ideas?