Ok so I think I figured it out. Here's what it says on the Pandora website:
"If your car stereo has an option for AUX input, you'll need an audio cable with a 1/8" (3.5mm) plug on both ends in order to connect the headphone port on your phone to the AUX input jack on your car stereo. You can then listen to Pandora on your phone when your car stereo is in AUX mode."
Since I can only plug usb in my car stereo, I looked around a bit and found this little connector piece on amazon for $5
http://www.amazon.com/3-5mm-Female-Stereo-Microphone-Adapter/dp/B0032AEF42/ref=sr_1_2?ie=UTF8&s=musical-instruments&qid=1280831820&sr=8-2 so I'm thinking that if I connect a 3.5mm jack from my phone to that connector piece, I should be able to plug the usb end into my stereo....

I'm going to try it, hopefully it works! I'll let you know how it goes, just incase anyone else is having the same problem.