ATT came out with their unlimited voice and text plan about a year ago and it's pretty cheap. I pay for 1.5GB of data, but they also have a 2.5GB plan. If I reach 60% of the data plan limit, they send a text and if I go over, they add 1GB, for $15. 2.5GB/month comes out to about an hour of Pandora every day but if you're on a WiFi network, that reduces the use of their in-network data use. I pay about $70 and have an iPhone.

FYI- My van's stereo doesn't have an AUX input, so I use an RF transmitter that plugs into the ear plug jack. The one I have is on the phone, not a long cord and I have found that it disrupts the cell phone's signal.