In the BOSE headunit in my Avalanche they have found the audio inputs into those radios from auxilary CD changers or the XM radio inputs.
You may want to look for a similar route. The big issue for the GM radios is that the radios are not programmed to recognize the port if the radio wasn't connected to an external changer or satellite and required reprogramming. I can do this now in my Avalanche by switching to an XM radio station. Splice the audio in and away I go, Apparently there is an auxiliary option also but with this easy solution no one on the avalanche forum bothered trying to figure it out. It seemed to be tied to the external CD changer though.