Have not done that, but you are probably correct on the harness layout. I have found extra connectors tucked away or found the the wires ran to the female side of the connector, You just have to add the wire from the plug to the seats. It's cheaper for Chevy to make one harness for various models than several different harnesses. Case in point: My 2013 Camaro 2SS had Boston Acoustics sound system but no subwoofer for sedan model. The same model in a convertible does has a factory sub. I traced the wires to the rear connecter and added them to my sub amp. So now the subs are directly controlled by the factory head unit and not tied into the rear speaker wires as most aftermarket sub amps are wired. Sounds really good.
Just Google the wiring connections for powered seats and then check for power on the corresponding pins. You may also need to add fuses that may not be populated. in the fuse boxes.