I belive radio shack (and plenty of other places online) also has an auto-switching switch that will automatically flip between one of the two inputs depending on which one is on. I'm not sure what it would do if both inputs were "on", but it might be worth looking into. It's more expensive, but could be hidden in the trunk/under a seat. Having a manual switch box that you have to press the buttons to pick an input is cheaper, but quite ghetto IMO.
found a link to an auto sensing one:
http://www.radioshack.com/product.as...%5Fid=15-1988#
sounds like it switches to the device "most recently powered on". Wonder if there's one made for car audio (or better yet, built into an amp). This one needs AC power and has TV/video crap.