I haven't done this but have considered it. The 1st paradigm you have to break is thinking that the dash radio has to be connected to your mp3 system. It doesn't. You can leave your in-dash AM radio completely stock. You can install a completely separate amplifier and speakers for use with your mp3 player. Any competent stereo shop will be able to do this, or you can do it yourself at minimal cost.
$50 - $100 for the amp
$50 - $300 for speakers
$100 for installation
regards,
Leon
