I just have a big USB flash drive plugged in and left in the glove compartment. That works great for me. I'm guessing power shuts off when it's powered off. Shouldn't be a problem leaving it plugged in all the time, just keep it hidden.
I've got a 2015 ES, so it does pretty nicely with voice commands. Me: "Play Artist Treacherous Orchestra." Car: "Did you mean <robot voice>Treacherous Orchestra</robot>?" Me: "Yes."
Sometimes, I'll get bored. Me: "Play Artist <insert nonsensical noise here>." Car: "Did you mean <robot voice>name of artist I haven't listened to in awhile</robot voice>?" Me: "Yes." Good as random.
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View my fuel log 2015 Mirage ES 1.2 manual: 44.1 mpg (US) ... 18.7 km/L ... 5.3 L/100 km ... 52.9 mpg (Imp)