ECM Power Source Circuit [12/2015 - 08/2016]: Description
WARNING: This page is about a different car, the 2016 Toyota Sienna. However, it is still accessible from the selected car via links, so may be relevant.
When the ignition switch is turned ON, the battery voltage is applied to IGSW of the ECM. The output signal from the MREL terminal of the ECM causes a current to flow to the coil, closing the contact of the EFI MAIN relay and supplying power to terminals +B and +B2 of the ECM.