Finally, someone like me!
It wouldn't be such a huge issue if the Bluetooth audio wasn't substantially lower in volume than the Radio or CD output. I use Android Auto in my Mirage, and if the USB cable disconnects for any reason (which I was having trouble with a specific model of phone randomly doing so), the head unit would immediately switch to the next input, which is the radio (and blast me to death).
No, there's no way to fix this. I've just made it a habit to always turn off the radio before turning the car off, so it's never on when I start it. I've found the hard way that when the head unit is still booting up (Mitsubishi logo on the screen), you actually can't adjust the volume or even shut it off, so I hope you didn't have it turned up previously!
One trick you might try is burn a CD with nothing but silence, and put it on loop. Hopefully it'll switch to that input first instead of the radio.
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View my fuel log 2017 Mirage GT 1.2 automatic: 37.5 mpg (US) ... 15.9 km/L ... 6.3 L/100 km ... 45.0 mpg (Imp)