Please note that the FM Transmitter app will NOT work on a T Mobile SGH-M919. You did not mention the make and model of your car so I am not sure if you can use Bluetooth or even the headphone out to Aux In. Alternatively, you can pick up a Scosche FM Transmitter at walmart and run from the headphone jack of the phone. I use a cheap Scosche Tunein FMT4 that sells for 14 bucks at walmart in the automotive section and it works great. I don't use Pandora though as the sound quality is sub par compared to the 128k and up streams I listen to on the TuneIn Pro app.
Current everyday device: Samsung Galaxy S4 SGH-M919 rooted, modified stock.