Pandora app always running while in car w/ Sync technology---want to turn it off!!

Hi--Everytime I am in my car, a Ford with Sync technology, the Pandora app on my iPhone 5 will start running and playing music, though most of the time it does not play through my car speakers, and I have closed out the app completely so that it is not even running in the background on my phone. I have scoured the settings, phone and Pandora, to try and figure out how to turn OFF the Pandora app, unless I specifically turn it on, but I cannot find anything. The only way I realize that it is playing half the time is because I can faintly hear music from my phone speakers, or the lock screen shows Pandora playing. This is not only annoying, but it is using up my data plan, is there any way I can fix this?

Thank you!