I've got the Discovery Nav, rather than Nav Pro but when I had my iPhone 4S, if I pressed and held the home button on the phone to invoke Siri the voice control all worked through the car bluetooth. I didn't have to do much to set it up - as long as the stereo was set up to handle both call audio & music audio, the Siri commands would just interrupt whatever was playing on the stereo at the time.
One thing that might be different is that I don't have voice control, whereas the Nav Pro does - maybe this is getting in the way of the voice activation of Siri ?
Annoyingly, since I changed my phone to an Android one, I can't get the voice control for Google Now to work in the same way as Siri did with my iPhone.
