navigation sound

When the navigation voice come on turn up the volume, it will turn the volume up for the navigation. Same goes if you try to use the navigation and the voice come on and you try to turn the volume down. It will not make the volume for the radio go higher or lower, just the navigation, but ONLY when the navigation come on.

It seems there is no relationship between the radio volume and the Nav volume. I'm going nuts trying to figure out how this poorly designed, ill-explained instrument can be heard above the din of the road. It is pretty much worthless as is. The commands are almost impossible to hear.....Whoever designed this one should be sent packing...