Since trial fitting the cross brace, and running the wires for the power steering, I managed to position the rear wiring loom and fix it into place.
I also had to run both fuel pump wires along side which makes it a little bulkier. I fixed the loom up under the passenger footwell using a P-Clip and then proceeded to duct tape the rest of the cable in place as it runs to the boot area. This now enables me to wire the rear lights and properly dress in the wires.
I also wired in some of my additional wires into a new 9 way round plug, and I had to colour code them to ensure they don't get mixed up.