Which wire is the ground wire and where is the ground location for the factory radio?

I put this drawing together to show the plugs on GM vehicles. The black plug has a switched 12-vot wire to turn the radio on, and a memory 12-volt wire to maintain the clock and station presets.

Your truck is right in the middle of the changeover from the three larger plugs they used through the 1980s. Those have four wires in each of the three plugs. If you have that style, they added a fourth, two-wire plug, clipped to the back for the memory 12-volts and the display dimming on electronically tuned radios. As I recall, the orange wire in that plug is for the memory 12-volts. I'm still looking for my drawing for that style plug. I'll post it as soon as I find it.

Here's the plugs for the older GM radios.