The installation at this point is just a matter of removing the dummy plug, plugging in the decoder, positioning the parts as shown and reassembling the shell.
Be sure the 8-pin plug is oriented and positioned so the orange wire plugs into pin P1.
I chose to locate the decoder and speaker in the center section over the motor cover and route the harness so as to avoid blocking the LEDs for interior lighting on the light board. I secured the speaker to the light board with a drop of liquid electrical tape or similar type adhesive.