The first step to re-assembling the gearbox is to make sure the wiring is in the right place. Check the switch itself is in position and screwed down.

If the AEG is wired to the rear then it should look like it does in the first picture.

Check to see if the switch spring is attached to the post on the gearbox and the trigger plunger.

Make sure the red wire going across the area circled is on the bottom.

Now you can add the trigger make sure the spring that fits on the trigger is attached. Without that spring the trigger will not be able to reset after a shot on semi-auto.