Does anyone have a Sony MEX B3800U and a HTC desire?

Does anyone know if you can stream the nav commands to the stereo via bluetooth/usb so that they 'interupt' whatever you're listening to (be it usb, cd, tuner etc)?? I can get the phone to stream music but the first time the nav cuts in, the music stops and doesn't resume; does anyone know a way around this as it would be better than nothing?

I can also stream the nav commands alone, by selecting the 'BT Phone' option but Ideally i would like to have the commands interrupting music rather than driving along in silence most of the time!!

I have also tried using the usb but, when it's connected and i try to load google nav, the phone says i need to install text-to-speech. This is rubbish as i already have this installed and it works fine when the phone is unplugged. Has anyone else had/fixed this problem??