Audio / sound injection for in car computer

Hello.

I'm working on installing a computer in my car. I built a 12V Regulator using parts I had around the shop but I'm stuck on the audio portion. I want the CarPC to be able to inject sound over the speakers from GPS navigation software without having to use an axillary input.

I've never worked with audio circuits before and I don't even know where to start.

I want to install an audio injection device between the radio and the speakers that will do the following:

1. Mute or better yet just decrease the radio volume to where it is low enough to be heard but not too loud to where the navigation audio can't be heard. I want the volume to decrease as soon as it senses audio coming from the carpc.

2. Inject the gps navigation sound.

3. Increase the volume of the radio back to normal.

I'm guessing it will need some type of delay before increasing the volume back to normal as there may be slight delays in between navigation audio.

I'm sure this is something easy to do but I just can't find anything anywhere on how to do this.

Can anyone give me any pointers?

You need to have a muting function and auxilliary input on the radio.
Or you need to design an audio amplifier with 4 or 5 channels with a muting function and radio and GPS inputs.