If I read correctly you search one switch automatic drive with the Line In?
Is one interesting question profit for the one that the D-Link DRU100 for instance uses and has to pilot an electric spar from car with the Line In to have a good receipt (as I have done me) however with manual switch
If you have a carpc then more than likely it has usb ports somewhere on it. And i know they are usually not readily available. Thats why most people locate a hub somewhere that is. And i have personally never seen a mp3 player that doesnt have a usb connection on it. I would use a usb hub somewhere on or near the dash for external mp3 players and then line-in/auxillary could be used for your fm radio. And with the usb hub you have almost endless possibilities for connections. IE> digital camera/camcorder, external hard drive, thumb drive, bluetooth adapter, etc.