Quote:
It switches not with a button, but based on which device is sending audio.



Really? My understanding of it is that it is controlled by 2 12V inputs typically connected to the head units remote wires,
with one of the inputs having priority if both inputs are active. Regardless of any unit actually sending audio.
(Which looks to be just what you describe later on) For manual control, just wire the control leads to a switch/switches instead.

There are circuits that will switch due to actual audio signal, located one for a guy on another forum some years ago for
getting GPS voice prompts over the main speakers. Downside is that it takes a fraction of a second to react
and switch over, but that was OK in his case as the prompts started off with an 'attention signal'.
IIRC it was a commercial 'assemble-it-yourself' kit. VOX source switch into google ought to bring up something...
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/Michael