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It's better to get the under hood body harness too. It attaches to the engine harness in 3 places. 1 is the alternator and there are 2 plugs that go to the main relay box under the hood. Also, the SC400 uses a 160 amp alternator and the relay box contains the 150 amp relay you need. It can be bought at an auto parts store but if you can get it, do it.
Get the fuel pump ECU if you can too. It's located behind the drivers side rear kick panel.
Finally, get the plugs for the ecu to go to the interior body harness. Try to get as much wire with them as possible.