Question for those with Zumo and Scala headsets that also play audio thru radio aux jack...

When Zumo is connected to radio thru aux jack and also blue-tooth connected to Scala headset, what audio gets sent to radio and what goes to Scala headset? Does it send all audio (directions, phone, MP3) to both? I'm hoping MP3 goes to radio and rest goes to headset, but want to check out before buying.

Thanks in advance for help.

Bob