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.
No batteries are required, it's sonically transparent and most audio gadgets have their own volume control so the mixer can be done very simply and cheaply.
This little mixer also works great for connecting multiple computers to one set of amplified speakers and has many other possible uses. Note that this device is NOT RECOMMENDED FOR HEADPHONES!!!! Whatever you plug into the output needs to have its own amplification, or the volume will probably be too low.
Note: Soldering is required. If you don't know how, please search for Instructables on how to solder, as that's beyond the scope of this project.