For what it's worth I started to fit the "genuine" harness today. The easiest way to get to the 4 in plug up near the top of the firewall is to unplug the ECU, take out the 2 bolts securing the ECU mounting braket, and remove the ECU. You can then get to the plug with ease. The top of the ECU bracket is up behind the dash, and has 2 rubber bushes that go into sockets. You can see them through the dash (with the glovebox out).
I think the harness they supplied me was possibly for an MQ (I have an MN). The harness has an 18 way plug/socket combo that is supposed to go in between an pug under the ECU. After searching for it for nearly an hour, and trying every plug I gave up, and cut it off. It is only to pick up the brake wire, and I will do that via the kick panel method. I checked out the rest of the harness connections, and they all go where they are supposed to go. The instructions are dated 2015, hence my belief that they are for an MQ.
I probably should have sent it back, however, the first one they sent got lost in the post, and I need to have electric brakes fitted by Monday. Apart from the brake wire, it plugged in OK. Nothing a bit of soldering couldn't fix