Dunno on the GMT900, but for the GMT800 the harness in the roof, above the headliner, has the Onstar, the XM, the clearance lights, and I think the Map Light wiring all in one big bundle...IF you have all those options. My truck for instance, no OnStar, no clearance lights, so my harness is map lights and XM only. I have a couple options, I can buy a clearance light only harness and tie it to my existing one, or I can buy the clearance light + XM + map light harness and change the whole thing. F-THAT. IIRC the clearance light harness plugs into a junction box on the passenger side, I think I remember reading that.
Looking back and digging, I *think* the stand-alone clearance light harness for a NBS truck is GM Part No. 15805944 Maybe someone can verify that for me.