Circuit/System Description
WARNING: This page is about a different car, the 2017 Chevrolet Spark. However, it is still accessible from the selected car via links, so may be relevant.
The rear seat belt switches each supply a ground to the SDM when their seat belts are buckled. When a seat belt is unbuckled, a switch opens removing the ground to the SDM. The SDM will then supply voltage to the seat belt indicator which will then cause that indicator to illuminate.