There's a similar thread where an owner could not find an open "harness" in the trunk. But noted that there was what looked like a factory stereo amp and a small fuse box on different sides of the trunk. If it is in fact the factory stereo amp, then it would be very easy for GM to make a harness that splits the appropriate signals off for use with the MTX sub upgrade.
2010 2SS/RS CGM
American Racing Headers, Magnaflow axle back, Roto-Fab CAI, Audio Integrations Sub box, Alpine Type R 10, Alpine MRP-500 amp, Pro Clips iPhone Mount