I did a lot of research on car integration a few weeks back and here are the only options that actually work:
1. KCA-iP500 (from Kenwood) - This is a little black box that you install directly into any kenwood head unit (even OEM headunits from car companies that are branded by Kenwood). You can control the iPhone with the remote and the song tracks scroll across the face of the headunit. It has a generous cord length so you can make calls even when it's plugged in.
The one drawback to this device is that if it's plugged into the system you MUST use the headunit or remote to control it. The iphone automatically blocks access to the iPod section of the iphone. Besides this, it works perfectly while connected, you can view maps, make phone calls and take photos. This was also the cheapest option. It cost me about $40 on Amazon.
2. DLO Transdock (or transdock micro) - This is an FM transmitter, and the only one that I tested that actually works good with the iphone. It has a nice built in cradle that charges and transmits and Unlike the kenwood ipod integrator you can freely control the ipod section directly from the iphone screen. It also has numerous other features like USB in and Auxillary in/out. This means you can integrate any other device that has a headphone jack. And you can pipe out video to your car's video system if you have one.
The drawback to this device, is that you have to rely on FM signal unless you have an auxillary in on your car or an rca adapter in the back of your headunit.
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