Back-up 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 back-up power source circuit for the A/C amplifier is shown below. Power is supplied even when turning the engine switch off and is used for diagnostic trouble code memory, etc.