I'm trying to connect my custom created navigation to my car audio. This like the way carkit systems do. So for example, I'l listening to radio, then when the navigation makes any sound (f.e. direction information), it should reduce the radio sound and play the navigation sound.

I know Parrot has a nice solution for this via a Blue box, bot this of course is only for parrot products. Are there any other ways to do this. For me it doesn't matter if I connect it over aux, bluetooth or hdmi.