Instructables user richfiddler11 writes:

My car stereo has only one ‘Aux’ input, but I have several gadgets that I like to hear on a trip without swapping cables: MP3 Player, talking GPS, Blackberry, XM player, Kindle, etc.

I couldn’t find a commercial solution, so I came up with this little passive audio mixer that takes 1/8″ headphone outputs from up to four devices and safely mixes the signals into one line that can feed a car stereo Aux input, powered speaker or any other amplifier that can take 1/8″ stereo output.