The newer cars have it but its pretty expensive and hard to do with the older cars. Do a search for combo box.
The cheap way out is something like the blackberry gateway or there are some other threads on other similar adaptors that play through the aux
I believe the Mobridge unit has A2DP and the Dension gateway 500 may offer it as an add on . Both are pretty expensive just to add USB and A2DP when you consider these come along with bluetooth phone on most aftermarket decks in the 250 + range .
KOS-A300 never caught on the reviews said it had problems.
search=a2dp returns a lot of stuff.