Wouldn't it perhaps be simpler to write an app that mirrors the phone screen onto the dash? That way people can use whatever navigation app they feel like, and you don't need to worry about maintaining the app on top of everything else. Just a thought.
Would love to do it this way - unfortunately Apple quite heavily restrict what developers are allowed to do on an iPhone, so the iPhone version of the app will operate much like other Map apps do. Android phones are more open - The other issue is the Bluetooth connection, to mirror the phone's display would require a much higher speed connection (e.g. Wifi), The bluetooth connection can't handle as much data so the directional updates over bluetooth will be very specific updates, e.g. "In 3 Miles, Left Turn, Distance to Destination, Arrival Time, Traffic Delays, etc.." - it will be very similar to what KTM have done with their 'My Ride' app which essentially does the same thing with their latest TFT Dash.
The other way to do Navigation would have been to do everything on the TFT Dash, e.g. give it a GPS module and have all the maps loaded onto the device - essentially what TomTom and Garmin devices do - however going down this route will then involve getting hold of the raw map data, creating the satnav software, and also providing map updates as the data gets out of date - with that in mind the smart phone route will be the quickest and the actual navigational mapping will be done with already proven and tried technology. Hope that helps!