> And that bus needs to be able to reach the audio system to provide audible feedback to the driver when parking, and also mute the radio. Ergo, you have a bus that reaches both brakes and radio

A $2 speaker would work just fine. You would still hear it over the radio. If your radio is so loud you couldn't hear it while parking you have bigger problems. A shared bus might be more efficient, but it is not more redundant. I would rather spend $2 on redundancy here.