The cleanest option is a cassette adapter if your car stereo has a cassette player, because you avoid loose cables and it works pretty well. If you don't have a cassette player, you'll need a 3.5 mm aux cable connected to the AUX input (if it has one) or to the microphone, though the latter sounds worse. There's also the option of an FM adapter that broadcasts the music signal over radio, but the quality isn't great and it can pick up interference. Any brand aux cable works fine, you don't need anything special; what matters is that it's decent so you don't lose sound.