You are right, the bulb cover is intended to rotate down the slot of the dome light body to push down the globe and switch on the bulb. The cable goes to one of the screws on the bridged connecting marked "tail".
There is no "parking" choice. Current to tail and instrument light is only on when the switch is on "dim" or "brt" but the horn is hot all the time, its wire goes to the center screw.
You connect the parking light to the lo beam light. That way there is a faint light even if the lo beam filament expire. And remember! The lights on, even daytime, is way easier to notice for other road users than with the lights off!
Current return can be troublesome with paint at many joints. Safest is to route a separate ground cable back to the instrunent dash. I soldered a ground cable from the bulb holder to the light body because there is often enough issues with the connection between bulb holder, reflector and light body.
