Have the scroll wheel control all sources together – such as music, navigation, etc.
Have the volume scroll wheel be a ‘master volume’ control. Navigation volume should be relative to that and set in the navigation settings screen (or on the map).
Current behavior: I tend to lower my volume as I approach my destination (due to lower wind noise at lower driving speeds). However, it often seems to happen at the same times as getting navigation prompts. So the scroll wheel turns down the Navigation prompt volume very low, and now I can’t hear it, and my music is still at high volume.