I looked for the service documentation in the past, and it can be purchased here:
http://www.helminc.com/helm/Result.a...ECSGBPQDXF0TV3
it's not cheap but probably worth. I didn't get any but may order the printed service manual some day when I'll need it. Not sure if that's what you need, but I doubt that you'll be able to find better reference.
Also the
http://www.gmparts-direct.com/ has schematics diagrams too but not very good quality.