Automatic Audio Source Switching
If I understand correctly, you have multiple sources for your car sound amplifier, and you want certain communications radio outputs to automatically override the main music source (the &quot;head unit&quot;) in feeding that amplifier.
One way to start is to make all those sources reasonably compatible. You can find &quot;speaker to line level output&quot; converters inexpensively. This will convert your head unit speaker outputs to line levels, to roughly match the comm radio outputs.
Now you need to design a circuit that accepts multiple line level inputs, senses audio on some of them, and automatically switches that input to the output. The sensing circuit will need a diode and capacitor to convert the average audio level to a DC signal. You'll want control over the sensitivity (creating a squelch threshold), and time constants for turn-on and turn-off.
You might be able to do this all in the analog domain but it would be more educational for you to use a microcontroller like an Arduino at the heart of it (search Arduino and find tutorials on microprocessors). You will also need to learn about voltage regulators and filtering to power the microprocessor in a 12V auto system.
The microprocessor analog inputs can do the sensing. For switching the audio, research &quot;CMOS Analog Switch&quot;. Create an initial design and bring it back here for review. It would be a very educational project.