The Drone wrote:

> The car and its radio are three years older than the phone and car kit.
> I had been told that the car kit muted the stereo but it doesn't appear
> to. I suspect that if I got an appropriate modern stereo, it would have
> a feed from the car kit to do as you say.

If the radio has no mute, it is a somewhat trivial matter to install a
relay that cuts power to the radio when the phone rings.