Here is an article I came across in PC magazine. Many of you have iPods and have wondered how to get all that great music off the iPod and playing on your car stereo. Here is 6 way to achieve it and the pros and cons of each.
1.) FM modulator.
The modulator is a low-power FM transmitter that broadcasts on an unused frequency. You plug a connecting cable from the modulator into your music player's headphone jack. Most of the portable satellite radio tuners from Sirius and XM have modulators built in, and usually run $20-$30.
Pro: Works on every car with an FM radio. Choose from multiple frequencies. Affordable. Most don't need installation.
Con: Quality ranges from okay to poor. For models that don't plug into the lighter, you must remember to bring a spare AA or AAA battery.
2.) Cassette adapter.
Plug this familiar (since the 1980s and the first portable CD players) gadget into your car's cassette slot, and press Play. It works adequately if you, ah, still have a car cassette player. Cost is $10 to $20.
Pro: Works with every car with a cassette player.
Con: Works only with cars with a cassette player. Adequate sound quality.
3.) FM modulator (antenna direct connection).
This modulator transmits, but not over the airwaves. You insert the car radio antenna plug into the back of this modulator, and the modulator's antenna plugs into the back of the car radio where it directly injects the signal. Sound quality is better, and less variable, than with other FM modulators. Cost is $30 to $75 and you may want an audio shop to install it. Check out products from Fahrenheit and Scosche, among others.
Pro: Adequate to good sound quality. Draws from car power (no batteries).
Con: May need pro installation (but it's not that hard). A handful of esoteric cars may lack the standard antenna jack or it will be in the trunk (requiring extra wiring).
4.) Line-in connection.
Run a cable from the headphone jack of your music player to a jack on the front of your car radio, a music-in jack in the glovebox, or in the center console. Can't find it? Automakers only got religion in the last year or so, and a few started adding line-in jacks, although others, like Lexus, will charge extra for it. Before, they (or the dealers) thought you'd be willing to pay $500 for a trunk-mounted CD changer that would play six CDs. Look for a stereo line cable with 1/8-inch jacks at both ends; this is the common size for music player headphones. Cost is $2-$10.
Pro: Excellent sound quality.
Con: Few cars have line-in jacks yet.
5.) Line-in connection via CD changer.
Most cars built in the past decade have a jack in the trunk for the dealer-installable CD changer you never bought. Or a CD changer connector on the back of the factory radio. If your car lacks a line-in jack and you want best sound quality, this is where you make the connection. Sound quality won't be any better or worse than a standard line-in connection, but this may be the only way to hack in. Some trunk-mount satellite radios can actually pass track-and-title information to the radio head unit. Cost varies from $25 to $150 plus an hour or two of installation time. It may be possible to have both a satellite radio and a music player connected with a custom A/B switchbox. Look into products from blitzsafe.com, pac-audio.com (Pacific Accessory Corp), and soundgate.com.
Pro: Excellent sound quality. Possible to pass satellite radio station, song information to car radio.
Con: Not cheap. May require disconnection of CD changer if you have one.
6.) iPod smart-cable connection.
Cables or black box (actually silver box) modules specific to Apple's all-conquering iPod connect into the CD changer jack or the back of the radio's CD changer / satellite radio jack. They pass the signal to the radio and charge your iPod at the same time. Some modules pass artist, track, and title information to the car radio. Others (Harman/Kardon drive+play, Monster Cable iCruze) have their own small LCD panels that mount on the dash, replacing the display of your iPod, which can be safely stashed in the glovebox. Costs range from $100 to $250.
Pro: Excellent sound, battery recharge, (sometimes) song information passed to car radio or dedicated LCD panel.
Con: Costly. Complex (for you, not the shop) installation. Disables iPod faceplate/controls, May limit you to a half-dozen playlists with specific names.