System Diagram
WARNING: This page is about a different car, the 2003 Lexus ES 300 and 2002 Lexus ES 300. However, it is still accessible from the selected car via links, so may be relevant.
The cruise control ECU is integrated with the ECM. With this system, the ECM directly controls the throttle valve to operate the vehicle at a fixed speed because it has adopted the ETCS-i (Electronic Throttle Control System-intelligent). See Fig 1.