With Windows Phone 8.0, I was able to connect to my car's bluetooth system in order to send texts. For this to work, I had enabled the "Use an alternate Bluetooth audio connection for Speech" option. When in the car, I was able to press and hold the start button, and the prompt would come over the speakers, and the cars built-in microphone was used to issue voice commands "Send Text to [insert name]". Having to actually press the button on the phone is less than ideal, but my car doesn't offer a way to trigger this from the steering wheel, so I could live with that.
Unfortunately, this functionality seems to have regressed with Windows Phone 8.1 Developer Preview. I was hoping to be able to activate Cortana by pressing and holding the Search button, but things don't work correctly. When I do so, the "blip" sound does come over the car's speakers, however, neither the car microphone, nor the phone is able to hear anything. I have tried connecting with and without the "alternate bluetooth audio connection" and the results appear to be more or less the same.
I haven't yet been in the car when I received a text message in order to know whether or not that functionality still works (being able to read the incoming text message and respond). Has anyone else experienced issues with Bluetooth in Phone 8.1?