Recently, I drove along on the Autobahn and listened to an audiobook. Suddenly, I got a slight crave for one certain song - but I didn't want to stop listening to the book. Because my phone can't (comfortably) play two audio files at once, I needed two devices for this job. But even when I gathered two audio devices (a phone and an iPod), my car radio has only one input port.
The initial question was "How do I get a second AUX port for my radio?", which quickly evolved to "How can I combine the output from two audio devices onto one cable?".
The usual solution would be a mixer - a bulky and expensive device. I didn't need the large crossfader, or even volume control - both my phone and my iPod can do this on their own.
So, after a bit of google-fu (and a lot of experimenting with resistors), this is what I built:
The blue part just amplifies the signal from the phone by a factor of 20 - a luxury that allows for resonable volumes without cranking up both the phone and the car stereo.
Of course, you'll need two of these to get stereo sound.
If you're going for the least effort, this would be your prefered circuit:
This version (again, you'll have to build two of those) doesn't even need a supply voltage. If you don't have 220µF capacitors lying around, preferably go for something bigger - everything below 100µF will muffle your base sounds. The polarity of the capcitors doesn't matter; they're going to be driven the wrong way half of the time, no matter the polarity. Voltage rating allows for everything above 3V.
The cover image shows the final circuit, built directly into the radio chassis (there was a lot of space after I removed the cassette module)