Circuit/System Description
WARNING: This page is about a different car, the 2008 GMC Sierra, 2008 GMC Cab & Chassis Sierra, 2008 Chevrolet Silverado, and 2008 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
The low beam headlamps are controlled by the BCM through the headlamp low/HID relay, and the high beams are controlled by the BCM through the high beam relay. Both relays are supplied battery voltage and are energized when the BCM grounds the low side drive control circuits. The headlamp on switch and high/low beam select switch signal circuits are referenced from battery voltage by the BCM. When a switch is activated the signal circuit is grounded, and the BCM determines the switch is active when the signal circuit voltage is low.