Put the wire harness back in place by sliding the lower portion down into the slot in the gearbox, then pushing the top end back down. There is a round hole in the harness that corresponds to a small peg in the gearbox. Don't forget to hook the trigger plunger spring back onto its post.
Reinstall the small screw that holds the wire harness in place.
Make sure the trigger spring is attached to the trigger as shown.