I don't think the USB cord is going to allow anything beside charging of device in your car, not playback of media content (unless there is a file browser built into your receiver that will allow you to browse to a folder that contains media files, but still wouldn't allow streaming from Pandora in this instance).
I believe you will have to use the audio/headset jack on the phone with an aux cable running into your stereo face, assuming it does have a line-in input jack.
Otherwise, if you do have bluetooth in your car you can sync the phone to the bluetooth which will allow media content to play through.
CRICKCRITTER1, please don't stop the music! You will need to sync your device to your vehicle via Bluetooth in order to play Pandora. Click http://vz.to/2sWepnC for the steps on how to do this.
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