I just switched to Lumia 1020 / WP 8.1 / HERE from my old Pureview 808 with Nokia Here. My car is a Dacia Lodgy with a Dacia radio and I mostly listen to the radio rather to music from the phone.
For me, HERE on WP 8.1 sends the navigation voice via Blutetooth to my Dacia Radio. Music gets interrupted, (no fading whatsoever), I hear the navigation instructions (completely, no cuts, but way too loud in comparison to the radio), then the music comes back. It works. But it drives me crazy.
I usually only need like 30 % of all the navigation instructions (e.g. when they say "turn right in 300 m", that's perfectly fine for me, in most situations I don't need another "turn right now"), sometimes I have HERE on because I am driving to an unknown destination, but I know 80% of the route that leads there and only need it on the last bit of the route, etc.
From my PUreview 808, I am used to a behaviours where phone calls get put through via Bluetooth, but the Here navigation directions come from the mobile device itself, not meddling around with the radio audio at all. It might have been a bug, but I liked that behaviour because I could continue to pay attention to my favourite radio show or listen my favourite music in those situations where I do not really need the voice instructions.
So what I would like to have is a configuration option to use phone speakers for Here navigation even though the phone is paired via Bluetooth in Driving mode. This would also be a good workaround for you guys who have problems with getting the navigation voice through to your bluetooth device.
Generally I feel that an incoming phone call is a higher priority event as compared to navigation instructions. That's why I want the navigation instructions to be floating nicely and non-intrusively in the background, whereas a phone call is allowed to interrupt my listening experience.