First, you may have to have "Use an alternate Bluetooth audio connection for speech" checked under Bluetooth -> Advanced... My VW's radio worked better with it checked, your results may vary.
To use Cortana, once connected to your car's stereo, hit the call button in your car, then say, "Call Cortana"... once the phone call is in process, you can say the same things you would as if you had just held the search button on your phone.... "send text to xxx", "shuffle music", etc...
http://www.windowscentral.com/how-to...-car-bluetooth
