Yes. You can get adapter cables for this sort of thing at a local electronics store or stereo store. You need a 1/8" stereo male jack on one end of the cable, and a stereo pair of male RCA jacks on the other end.

Remember, though, that headphone outputs are amplified (albeit with a very low-power amp). If the other device has a line-level output, you should use that instead. The headphone output should be used as a last resort only.

If you plug the headphone output of another device (say, a portable CD player) into the Rio Car's aux-in, start with the CD player's volume turned all the way down. Then slowly increase its volume until you can hear it well but don't hear any distortion.

Note that connecting to the player's aux-in might produce ground loop (whine/noise) problems, depending on what you're connecting and how things are wired in your particular car. If this happens, try grounding the shield of the RCA cables to the vehicle chassis and see if that takes care of it.