The switched 12 volts would have come from the OEM radio. Most (if not all) aftermarket radios also have a switched 12 volts output (for antenna amp, rear audio amp control etc).
I think most A2 are canbus, and the OEM radio only had a permanent 12 volts, with switching (on with ignition, off with key out) via that data bus. If you fit an aftermarket radio, you usually need to pick up a switched "accessories" 12, volts to control the radio, so it goes on and off with the ignition key, like the OEM unit did. I don't have any A2 circuit schematics, but it usually comes from the key switch.
Your need a permanent 12 volts to retain station programing, and I'm guessing this is present and correct. If anyone has a circuit we can find the accessories switched supply, hopefully the A2 has one ...
If so, you won't have to switch on and off manually. Fingers crossed.
Mac.