Headlight Signal Circuit: Description
WARNING: This page is about a different car, the 2006 Lexus IS 350 and 2006 Lexus IS 250. However, it is still accessible from the selected car via links, so may be relevant.
The main body ECU RH receives the HEAD position signal from the combination switch as a backup link. It also receives the signal from the combination switch via the multiplex communication system. If this link is grounded, the headlights will illuminate.