[Cliff8928]

The high and low beams are controlled by the switch. The BCM merely gets notification on which is currently active.

The BCM DOES control the automatic lamp control relays, but those merely provide an alternate ground path for the switch to turn on the lights. So no matter what, if the headlight switch is in "AUTO" and it's dark out, the ALC relay signal (dark green) wire will be provided a ground signal to turn on the headlights. You jump that over to the low beam (dark blue) and then your low beams will be on regardless of the dimmer position.