That's actually a picture of a lightweight locking retractor.
Your seatbelt locks at the latchplate (the thing that slides along the belt and buckles into female end) when a carseat is installed. Female and Male - think body parts.
You can try this with yourself... sit in the seat and buckle up. Pull all the slack out of the lap portion. Now try and pull straight up on the lap portion... it won't loosen. When the seat is buckled and the lap and shoulder portion are touching each other vertically (at the latchplate), they cause the latchplate to lock. You will be able to freely move the shoulder portion, but the lap portion will stay put.
It will be easy to install!
Put the Radian in place, feed the belt through the correct path and buckle. Then kneel in the seat and pull all the slack out of the lap portion of the belt.
A great Forward facing trick is to open the velcro on the side closest to the buckle. Pull some extra shoulder portion of the seatbelt through that opening (after buckling) and then kneel in the seat and pull straight up to remove all the slack in the lap portion. Once all the slack is out feed the remainder of the slack from the shoulder portion back through the velcro opening and into the retractor. Check for less than one inch of movement at the belt path. Attach top tether and all done!
If the lap and shoulder portion are not touching each other vertically, then you may have to twist the latchplate 180 degrees so that they are forced to touch each other vertically, causing the locking mechanism to work properly. This will result in a twist in your seatbelt (one at the lap portion and one at the shoulder portion) and is perfectly safe if you need to do this to make the lightweight locking latchplate function correctly.
Edited to add: for some reason Chevy has some model vehicles that do both... you definitely have a lightweight locking latchplate AND if you can pull your belt out all the way and it switches into locking mode then you ALSO have a switchable ELR/ALR belt. You can install using either locking method... whichever gets the tighter install.
You also do NOT have forward of the bight seatbelts, so I think you'll find it fairly easy to install the Radian in your vehicle.