The white part has been assembled into the metal body. If the aforementioned lug and recess aren't properly mated, the white part will be hanging out of the metal body a bit. Properly installed, the entire piece is within the metal cavity. The black part gets pressed into location, after which the black plastic cap gets snapped into place. It takes a good bit of force to snap the cap on. You don't need to hammer it or anything, just press it in with your hand. Reinstall the three small screws to secure the cap. Now you can start putting your bike back together again. As soon as you get the switch installed and connected, make sure your bike will start. Don't wait until after you've installed all the tupperware. I hope it went smoothly for you!