You need an FM transmitter, generally compatible with any kind of mp3 player. If you're lucky, you have an AUX in on your car stereo and you just need the proper audio cable to connect the iPad headphone output to your AUX in.
If you buy the FM transmitter, make sure that you can select the frequency to use, to avoid being stuck on a frequency already used by a radio station. You can get one for around 20$, but don't be afraid to get one around 80$, for better quality sound and better components.
VicoPad addict!