Start with a bored helper:
Populate the Mainboard DB37:
Populate the MS3 Expander DB37:
Acquire an OEM Harness connector:
Chop off the top pins so they are about 1/4" long:
Bend up the lower row so they are flat (try to actually fold them back straight):
Cut them flush to the top row:
Pre-tin the pins you'll be soldering wire to.
Put heatshrink on each of your wires. *SUPER IMPORTANT*
Pre-tin the wires.
Start soldering the Mainboard DB37 wires (I start from left to right, lower-rows to top-rows):
Follow up with MS3 Expander wires:
Melt the heatshrink so you ensure no wires will make a connection:
Follow that up with hot-glue so the wires stay secure and are less prone to vibration & stress:
You can populate the center connector for inputs and output, I used to do this, but lost my source for pre-populated connectors -- i hate crimping pins