This is what AD2P is:
Advanced Audio Distribution Profile (A2DP) is a set of standards for devices using Bluetooth, the same radio technology used in cell phone headsets and wireless computer keyboards, among other things. A2DP devices offer a one-way stream of high-quality mono or stereo sound, unlike the more familiar Bluetooth headsets, designed for phone calls, that use a two-way connection.
Unlike older technology for wireless headphones, A2DB is digital; the headset is "paired" with the device sending, so you can have multiple headsets in range of each other without cross-talk.
Which means you need a receiver capable of receiving the digital audio.
Parrot.com offers several excellent system that supports A2DP as well as direct connect. Other systems are available but in most cases standard hands free kits do not offer the standard A2DP
depending on make and model many cars offer iPod connectivity I suggest contacting the Car manufacture web site and see if they offer that option as it is by far the least expensive way to have digital in car sound. The fm transmitters offer it but use FM radio however I am yet to find something that works with out problems. The parrot systems cost around $ 300 plus installed mainly designed for mobile phones and digital music. I own the 9200 and use it every day with the iPhone 3. I also use the Bluetooth option for playing movies and games when in the car from my iPad when not driving lol and it is awesome
PS I am not associated with Parrot information provided as an option for in car sound.
Hope that helps