I'm going to try to anwser what i understood of your question , so i'll start with a MONO amp for a subwoofer. Run the power wire from the battery threw the driver side of the firewall along the floor runner thats along the side of the driver seat and rear seat. Run it along the bottom of the rear seat and out the back of the fold down chair. As for your groudn cable you need to find a clear spot in the trunk and sand it down so bare metal shows and use a self tapping screw to hold your negative cable down.
Now for the power wire so your amp turns on when you turn the ignition
if the have the amp on the rear deck unplug the connect facing towards the outside of the trunk "connector closest to you if your behind the car" and its the small gauge pink wire. I'll put a picture up in a minute. If you don't have the amp then i can't help you.
Car domain resized my image and forgot that text woudln't be readable anyways the blue wire is my 12 volt to the amp just ignore it its wired into the small gauge pink/purple wire.
As for the subwoofer signal i tapped into the subwoofer wire on the rear amp
This is a diagram i found as i'm not sure of its accuracy it was correct about the power wire.
2007 Chevrolet Monte Carlo Car Stereo Wiring Diagram
Car Radio Battery Constant 12v+ Wire: Red/White
Car Radio Accessory Switched 12v+ Wire: The radio harness does not provide a switched power source. Run a wire to the fusebox for switched power.
Car Radio Ground Wire: Black/White
Car Radio Illumination Wire: Violet/White
Car Stereo Dimmer Wire: Purple/White
Car Stereo Antenna Trigger Wire: Pink
Car Stereo Amp Trigger Wire: Pink
Car Stereo Amplifier Location: Under the rear deck in the trunk.
Car Audio Front Speakers Size: N/A
Car Audio Front Speakers Location: N/A
Left Front Speaker Positive Wire (+): Tan
Left Front Speaker Negative Wire (-): Gray
Right Front Speaker Positive Wire (+): Light Green
Right Front Speaker Negative Wire (-): Dark Green
Car Audio Rear Speakers Size: N/A
Car Audio Rear Speakers Location: N/A
Left Rear Speaker Positive Wire (+): Brown
Left Rear Speaker Negative Wire (-): Yellow
Right Rear Speaker Positive Wire (+): Dark Blue
Right Rear Speaker Negative Wire (-): Light Blue
Left Subwoofer Speaker Positive Wire (+): Dark Blue/White (at amplifier)
Left Subwoofer Speaker Negative Wire (-): Light Green/Black (at amplifier)
Right Subwoofer Speaker Positive Wire (+): Dark Green (at amplifier)
Right Subwoofer Speaker Negative Wire (-): Light Blue/Black (at amplifier)