The rear light gets wired to terminal #2 on the light switch. The front lights wire are wired to terminal #3.
Properly wired rear light will burn brightly in both the D and B switch positions; the headlights burn dim in D and bright in B.
That little coil on the light switch is the dimming resistor for the front lights. Be aware it becomes quite hot when the lights are burning dim - make sure no other wiring is touching the resistor when the switchbox cover is in place!