Mine works like this:
Black wire from engine to taillight side of bulb (the dimmer side.)
Ground wire attached to taillight side of bulb, to frame ground. (So black wire and ground wire attached to same side of bulb, dim side, always on.)
Brake switch wires: one goes straight to brighter side of bulb. Other spliced into black wire at engine. No ground.
Had dual switches, one on each lever, but one switch failed. When it worked, the switches were wired:
Front brake switch, one wire straight to bright side of bulb.
One wire from front brake switch to rear brake switch.
Other rear brake switch to black wire on engine.
No grounds in switch wires.
The rear lever switch on mine failed. It was old and crusty. Pulled both wires off rear brake lever switch and taped them together, so brake light work off front lever only.
So basically, ground taillight off power side, don't ground switches.