But it still mutes.
There is Mute which should mute completely.
ATT - which attenuates the volume slightly (but only works with the Pioneer amp and when playing media through the F500 itself)
Normal - which is what I thought would not MUTE the radio, however it does, so I presume this only means it will not mute the Pioneer amp??
It says...
Switching the sound muting/attenuation
You can select the muting method when the navigation guidance is output.
When you use a mobile phone connected with Bluetooth technology (dialling, talking, incoming call), the AV source volume is always muted independently of this setting.
When ND-G500 is connected with this system, the sound output from the connected car stereo is always muted when the navigation guidance outputs.
That last sentence made me think that when the G500 amp was not connected it would not MUTE if the right setting was chosen, however not the case.