I am not sure the Rio will work with the AUX/CD

I think that some (not all) head units will only let you switch to the AUX input if they detect the presence of the same manufacturer's CD changer. This "detection" involves icky proprietary protocol over the CD changer bus, which our car player isn't capable of faking-up.

You should check with your head unit's manufacturer, or with a knowledgeable professional installer.

Peter