ok I have just had a look at the spec sheet for the Omron G6KU-2P-Y. That's probably what I would use so I can handily solder it onto a perfboard.
If I understand its internal schematic correctly, I could connect both stereo sources (Bluetooth audio / cable connection) to just this one relay, and then let that relay switch between them as needed?
The thing is, it says it's got a switching current of 1A. That in itself is not a problem, as my whole circuit including the ESP32 will probably be hooked up to the car's cigarette lighter lead, which can tolerate some 8.3 A (the cigarette lighter socket is rated for 100 W at 12V). Since I don't smoke, the only thing that normally draws additional current from that socket is a plug-in USB phone charger at some 1-2 A.
But because the cigarette lighter is "always-on", even when the ignition is off, a relay that would constantly draw 1A would drain the car battery dead in just 24 hours.
So my idea would be to switch the relay so that in its "switched" state, the Bluetooth audio source path is open, and in its "unswitched" state, it goes to the cable connection.
Maybe allow a grace period of an hour or so after the ignition is turned off, and then the ESP32 terminates any live Bluetooth connection, and then unswitches the relay and goes into sleep mode, to be woken up again if a Bluetooth connection is reinitiated.