You may have been misled by the wiring diagram on the first page. A corrected version is attached for the 6 disc unit. This mod also works on my 2003 Chevy Suburban with Onstar, and should work on any GM vehicle with these head units in it. There was just barely enough room above the 6 disc changer ...

Here, get a cheap grounding strip and use these to cut the grounding prong off the end.