You need an automatic speaker switch. Such switches detect which source is active, and give priority to one source if both are simultaneously providing a signal, letting only one signal through to the speakers at a time. The ones I've seen need external power, so you would have to find one that runs on 12v. As I recall, the basic units aren't terribly expensive; maybe $50-$100, but it has been several years since I looked into such things. Check with your local Radio Shack or Frys, they may have them.
Never forget them. Do something to prevent it from happening again.
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Last edited by SlowButSteady; 03-18-2011 at 03:33 AM.