IG Power Source 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 power source is supplied to the A/C amplifier when the engine switch on (IG).
The power source is used for operating the A/C amplifier and servo motor, etc.