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Dude! I *hate* this, and have wondered the same thing. I've blasted my ears a few times. I've tried to manage it by only plugging in *after* I start the car -- I think that keeps it from happening(?) But sometimes I forget to do this.
 
It probably depends on which car you have, but I haven't figured out how to accomplish it with my car. It likes to start playing music when I plug it in. The one "solution" I came up with was to store a song called AAA on my phone that is just 1 hour of complete silence. Of course that only works when it decides to play the 1st song (alphabetically). Sometimes it picks up where it left off.
 
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