Do you want to play music from your iPhone, iPod touch or iPad through your car speakers wirelessly but dont have a Bluetooth head unit in your car? Now theres a solution with Apple AirPlay (iOS 4.2 and above).
What you need:
1) AirPlay compatible device: iPad, iPhone 4, iPhone 3GS, and iPod touch (2nd generation and later) with iOS 4.2 or newer installed.
2) Apple Airport Express (either wireless G or the latest wireless N will work the same).
http://www.apple.com/airportexpress/ ($99 new from Apple.com or as low as $50 on ebay used)
3) A car power inverter to power the Apple Airport Express in the car. The smaller the inverter the better. They can get hot. There are many different models and sizes out there.
4) 3.5-mm audio minijack male to male
Setup instructions:
1) Plug the Airport Express into the power inverter in your car. If you mount it in your glove box or center console keep an eye on how hot it gets.
2) The Airport Express is broadcasting a wireless network SSID right out of the box (If you buy a used one just hold the reset button until the lights flash) so no configuration is needed unless you want to change the wireless network name or enable wireless encryption. Youre just broadcasting music to your car so you probably dont want to go to the trouble.
3) Connect the 3.5-mm audio minijack
4) On your iPhone, iPad, or iPod touch connect the new wireless network (no encryption, no password). Go to Settings/Wi-Fi.
5) Open the iPod app. If youre connected to the Airport Express wireless network a new AirPlay icon will display next to the rewind/play/fforwad buttons for you to select the car speakers.
6) I was not able to detect any sound quality loss compared to plugging the 3.5-mm audio minijack directly into my iPhone.
7) Car Audio shops can help you hide the power inverter and Airport Express by running power directly from the battery etc to get the clean look of no wires depending on your car setup.
Update: Guess I should have used it a little more before posting this. One big oversight...When you're on wi-fi to an Airport Express that's not internet enabled in your car you can't browse the internet using Safari or use Pandora or Youtube or.....Basically it's cool to setup, but limited use. You have to go to Settings/Wi-Fi and disconnect when you want to use internet again in your car. Doh. Guess Bluetooth is still the way to go for now, but is the sound quality as good in your car?