The compressor is underneath the vehicle.
On the passenger side the Onstar box is there. Your onstar won't work even with a subscription because it's analog. I found this out when I tried to enter that "push the blue button giveaway" the other day.
However, you would need to do some serious research to take it out as you can't just remove the module because it's tied into the PCM and BCM. You can unplug two of the connectors to disable Onstar and keep the truck happy, but I am not aware of anybody completely removing it without serious problems.
JL does make a box that goes into the center console, you have to remove the factory sub but you won't need it anyways. Could put one sub there and the other where the jack is at.
A simple solution would be to build a box that fits the size of the floor behind the 3rd seat and downfire the subs. You would loose about 8-10" of height for cargo but should be able to get plenty of airspace and not have huge amounts of $$$ into custom fiberglassed boxes.