Is your iPod AUX in, or AUX out?
It might be tempting to buy one of those cassette adapters or a radio transmitter kit, just to avoid doing a lot of research. The cassette adapter consists of a cassette with a cable that plugs into your iPod, but you still have to pick up and hold the iPod to control it -- and it doesn't sound all that great. An FM or radio transmitter uses a connector on the iPod to send a radio frequency to your car's radio. There are several of these devices available through third-party developers, including the Griffin iTrip, the iCarPlay and the Roadmaster. Similar to the cassette adapter option, if you use an FM transmitter, you still have to control the iPod manually, and it'll probably sound even worse than the cassette adapter due to interference from other radio signals. FM modulators, on the other hand, have an advantage over transmitters. Instead of relying on surrounding radio frequencies, a modulator plugs directly into the back of your car radio, using its signal to play music directly through your car stereo. This eliminates static and makes the sound much clearer. The only issue casual iPod fans might have with this option is that you have to remove the radio unit entirely to install an FM modulator -- so it's definitely not the simplest solution for playing your iPod in the car.
Aftermarket or third-party adapters are generally sorted into two categories: vehicle-specific or stereo-specific. If your car has an aftermarket stereo (one that you installed yourself or paid a car audio company to install), begin by figuring out the brand and model and then look for accessories to match it. If your car still has the original factory installed stereo, then you should look for aftermarket connectors that are compatible with the brand of vehicle you have. Further complicating matters is that not every accessory is compatible with every model of iPod, so double-check before buying anything.
If you have an aftermarket stereo, especially an older one, you have a few good options. However, while newer aftermarket stereos are a lot more likely to have iPod functionality built right in, an older one will probably take a little research and perhaps some wiring skills. Check first to see if your car stereo has a USB input. If so, simply connect your iPod's charging cord to the iPod and then to the USB slot on your stereo, and you're done. Most car stereos, especially those with CD players, also have an auxiliary input jack, which works in a similar way, except you use the iPod's headphone jack instead of the charging jack. However, you'll probably need to buy a specific cable (it'll look like it has the headphone "male" end at both ends), and it'll probably drain the iPod's battery pretty quickly. These head units are usually designed so that once they sense an iPod is connected, you'll be able to control the iPod using the stereo's buttons and dials. Both of these options are simple to accomplish and will provide the best sound.
Another possibility is that you have a car stereo with neither an auxiliary input jack nor a USB port, in which case, you can install an external control box. This little module is installed on your car's stereo and it reads the iPod's signals and sends them to the unit. Once it's installed, you hook up your iPod as if you were using the auxiliary input jack described above, and then you're good to go. But you might need to end up buying extra wiring harnesses or other parts, and it's probably better if you have some experience with this kind of thing. It may even involve taking apart your car's dashboard.
While some options, like an iPod dock, allow you to charge the iPod while you listen, a patch cable only does the job of sending the audio signal to the player. If your vehicle is equipped with a cigarette lighter or a power port, both Apple and third-party developers offer chargers that allow you to recharge your iPod in your car. And if you're worried about your iPod sliding around your car and getting scratched, it might be a good idea to get a mount to keep it in place.