I have replaced my Becker Headunit with a new Nakamichi one (very happy with that improvement).
My Nokia carkit could mute the factory radio through its mute function (which was good so that my phone does not answer when the radio is too loud).
The Nakamichi headunit does not have a mute wire to interrupt it automatically. So I want to interrupt the accessory power (with a relay) to it so that the radio will simply shut off (same as when you turn off engine) when the call comes through and come back on when the phone is hung up.

Please advise on the installation of the relay. I don't know if the mute wire was a ground wire or a hot wire. I am sure someone here knows the answer.