Aha, yes, here it is. Super simple:

http://www.epanorama.net/circuits/linemixer.html

Scroll down to the part where it says "Can I make this circuit even simpler?"

This is what I did to combine three different devices onto a single RCA AUX input pair on my home stereo. Works perfectly without any noise or problems. Remember that the circuit shown is only one half of the system: i.e., it's just one of the two RCA plugs. You need to make a second one for the second RCA plug in a stereo pair.