Crutchfield is pretty good about informing the customer of adapters available for a CD Receiver with changer control to allow you to connect a normal AUX source. I searched the site myself and all I found was the XA-300 you mentioned, and one other that does essentially the same thing for the same price (XA-C30?).

This leads me to believe that there is no cheap, simple adapter to allow you to connect an auxiliary source to a Sony CD Receiver that doesn't already have AUX input. Or if there is, Sony doesn't make it. (Yet another reason not to buy Sony.)

A search on Google for "sony aux adapter" didn't turn up much, either. I know that other car stereo makers do this; Alpine for one. But obviously Sony saw the opportunity to create yet another expensive accessory.