Nokia N97mini + OviMaps + Bluetooth

Hi all,
I've had an issue with my car BT and I need to know if someone has solved it.

I own an european Prius that is like a Prius IV, so I have bluetooth with steering wheel controls but without navigation system.

First of all, my BT connection works well, I've paired my phone and I can easily make calls, receive calls and transfer the phonebook.

My issue is about OviMaps, the app made by Nokia that make your phone runs like a GPS navigatoing system.

With a common BT headset connected with the phone, the route guidance voice is transmitted to the earphones via BT, excluding the phone main speaker.

When the Prius is connected, instead, the guidance voice can't be heard. I think that the voice is sended by the phone via BT connection but the Prius system doesnt recognize it. So the speakers of both phone and Prius remain silent.

There's a way to solve this issue? I can deactivate BT on my phone, and so I'm capable of use the navigation through the phone, but I have to reactivate it every time I need to make a call or to answer it.

thaks to all, and sorry for my poor english!