Circuit/System Description
WARNING: This page is about a different car, the 2019 Buick Cascada, 2018 Buick Cascada, and 2017 Buick Cascada. However, it is still accessible from the selected car via links, so may be relevant.
With the headlamp switch in the ON position, the headlamp switch headlamp ON signal circuit is grounded through the headlamp switch. The body control module (BCM) responds by applying ground to the headlamp relay control circuit which energizes the headlamp relay. With the headlamp relay energized, the switch contacts close allowing battery voltage to flow through the headlamp fuses to the left and right headlamp ballasts illuminating the low beam headlamps.