RE: 2008 silverado dash panel
you you have the radio and hvac controls out, you have taken enough apart to remove the VCIM(onstar Module). There are two plastic clips on the left and right side of the vcim, relase those clips as you pry up the front of the module with a screwdriver. Its a bit of a pain but its do-able. Also you might try going in from the rear of the dash from the passenger side, and unplug the connectors. The should be anough harness there to be able to do your testing.
I must say though, that by removing the GPS antenna from the VCIM, not only will the red light be on, but if you were ever in an accident, onstar would not know where to send the emerency crews to come help you. Also if your truck is ever stolen, Onstar would not be able to assist the police in finding your vehicle.