Go here:
http://www.discountcarstereo.com/detail.aspx?ID=1467
They have an external audio input cable that connects to the rear of the CDR-220 along with three options for the player end connector (ipod/iphone, aux, or RCA jacks).
Installation instructions are on their site.
Please note that you will lose use of the factory CD changer (this is where the external audio cable plugs in).