Curiosity. This has been very interesting and has helped me validate some ideas I have had. Almost moving off topic now but I also found this circuit
So a circuit like this could be used to trigger the A-B switch instead of the HU remote Amp on signal. eg. A phone kit or CB radio could grab the audio output with no PIC involved when the speaker cackled to life! You would need to use a speaker to line out adapter. Ive already got one of these and it has attenuation controls built in. No longer using it since I put a car PC in.
This could be used to route both phone and my CB (using one channel each and splitting it).