1998 am/fm/cd delco radio Aux input

In my 98 monte carlo, I have an am/fm/cd delco radio with a 9-pin aux input for slave devices.  I want to set up for direct line input(from mp3 player) to the aux feature.  The gm9-aux [URL=" http://www.logjamelectronics.com/piegm9aux.html"]http://www.logjamelectronics.com/piegm9aux.html[/URL] is exactly what i want, except it says "does not work with am/fm/cd radios".  Is This True?

I recently came up with the idea of using a slave cassette player to enable aux, then splice the actual audio wires in the 9-pin harness.  Would This Work?  Does anybody know which wires these are?

Thanks