I am using a mini-phone plug adapter, but I've tried two different adapters, and at least one worked fine with a normal stereo. Even if the adapter was the problem, that wouldn't explain why noise comes out the left channel when no AUX input is connected. Nor would it explain why the music comes out of the right speaker when only _either_ the right or left Rio Car AUX RCA jack is connected to the CD or mp3 player.

I also tried looking for shorts between the right and left AUX signals, but they were both correctly floating with respect to each other, and to ground.

Thanks for the idea,
-Jeff