Are there any car stereos out there that support AirPlay? First, I understand that I'm walking a fine line but, to be clear, I am sticking to the FAQ and I am not asking for hardware recommendations.  Instead, I simply want to know if there are any car stereos out there that have AirPlay built in.  If not, why not?

This seems like a no brainer, especially considering that AirPlay has been around for a while.  Nevertheless, other than hack-it-together solutions, I can not find a means of getting AirPlay to work in my car.

Does anyone have any insight?  Is there a chance that I'm searching for the wrong type of product (i.e. Apple allows for car stereo manufacturers to integrate with iOS devices through some other means) and that's what I should be looking for?

Any tips would be helpful.
AirPlay works over UDP and it requires that both the sender and receiver device exist on the same TCP/IP-based network. That's generally not possible in your car because you aren't running an 802.11-based wireless network in that space.

For wireless audio streaming in your automobile you want to look for a head unit that  supports the Advanced Audio Distribution Profile (A2DP). This is a BlueTooth spec that lets two BT-enabled devices exchange high quality audio streams. Your iDevice supports it, you just need an A2DP compatible head unit and you can stream your music to your car system without any wires.

Happily, there are many head units that support A2DP on the market today, both aftermarket and OEM.