When a call comes in MUTE goes low, pausing playback. TELON goes high, switching the audio to TELNF+ and TELNF-. If you have a mute kit and a nav instruction comes in, it will disrupt whatever is coming from your headunit, whether it be CD, FM or Bluetooth.

My recommendations are twofold. First put the nav instructions through only two speakers, the front or the back, that way you get a mix of CD or BT with the nav instructions. Second put a 10k resistor in series with the new MUTE line to limit current flowing into the nav from the Bluetooth and damaging it.