ECM Power Source Circuit [10/2015 - ]: Description
WARNING: This page is about a different car, the 2019 Lexus GS 300, 2018 Lexus GS 300, 2017 Lexus GS 200t, and 2016 Lexus GS 200t. However, it is still accessible from the selected car via links, so may be relevant.
When the engine switch is turned on (IG), the battery voltage is applied to the IGSW terminal of the ECM.
The output signal from the MREL terminal of the ECM causes a current to flow to the coil of the No. 1 integration relay (EFI MAIN1 relay), closing the contacts and supplying power to terminals +B and +B2 of the ECM.