First, you need to label all of your electrical connectors form the engine bay donor engine harness. This is an especially important if you cannot find enough time tocomplete the wiring of a sub-system in a single day or afternoon. Because you will waste a lot of time if you have to come back a week later and have tore-trace each wire or determine the necessary color codes or wires tocompleting the sub-system circuit.
I strongly suggest that you spend the time to make a vehicle schematic with each wire’s color codedcalled out on the schematic to assist in the construction of a new wiring harness. Drawing a complete wiring schematic will save more time, then it takesto make it, Vs trying to figure out which wires are which after having been away from the build for a week or two. I made my wiring diagram by enlarging copies of the Toyota wiring schematic from a work shop manual. I then cutting out the ECU and other circuit sections and pasted themon to a 20” x 30” sheet. Then draw in the connecting wires with color codes indicated on the schematic. [Draw it in pencil]use the same wire colorcodes and wire gauges from the Toyota. Using the Toyota stock color codesmake the designing, drawing, and fabrication of the wiring a lot easier.
Using the same color codes as Toyota will make the installation and trouble shooting of the various sub-system plus any additional electrical components a lot easier to figure out.
Using the Toyota engine bay harness with all the ECU multi-plugs is the safest and easiest way to go.
You can never have too many electrical grounds for the main electrical components and accessories. Tap directly into the chassis tubes for your ground wires.
I would also add a few spare wires in the harness run and note the color code and gauge of the extra wires in the harness, for reference when needed at a later date for possible use for a fan or fuel pump relay. Another option is to just keep some of the un-used wires in the harness and note the color code in your log book.
You will also want to get the larger grommet for the fire wall, that the ECU plugs will pass through.
After you have completed the wiring, you need to check each completed circuit. Once you have verified that the circuit is OK, then move on to the next. Dave W