There are three primary way of transferring the audio from an iPod to the speakers. These methods are dependent upon factors such as: iPod model and compatibility, the car's audio head unit capability and the budget and preferences of the listener.
The first, most cost effective solution is use of a cassette tape adapter. These usually sell for $15-20 and plug into the headphone jack of the iPod.
The second is by way of FM radio transmission. Depending on the features involved, these units generally sell starting at $40 and don't go for much more than $90. These units plug into the 30-pin dock connector at the bottom of the iPod and send a radio signal to your car's antenna. The downside to using these is that it tends to take a while to find an empty frequency. Also, when traveling from city to city, some re-tuning may be required. There is no wire between the iPod and the car, unless the unit also offers charging capability. If there is no charging feature on the transmitter, this method will drain the iPod's battery fastest.
The third way of playing iPod music through a car stereo is by USB connection. Unless the car is of a newer model and already includes a USB capable head unit, an aftermarket head unit is required. These range in price from about $70 to over $1000. Most often, the iPod is controlled by the head unit, so changing songs and other operations would be done through the head unit's interface, not that of the iPod. The iPod would be charged by the car.
