Hello BeckSunny,
Thanks for choosing the Apple Support Communities. From the sounds of it, when you plug in your iPhone to charge using the USB port on the stereo, it is overriding the Bluetooth connection thinking you are trying to connect via Lightning/USB. If you change your channel to Aux or USB and test, does the music play via USB? We suggest giving that a try. Some stereo decks will do that when plugged in.
For more information on playing audio through your car stereo, please follow: How to connect your iPhone, iPad, or iPod touch to your car - Apple Support
We hope this helps.
Cheers!