[DannoXYZ]

Trace power along wires from brake-lever switch all way back to light bulb.

• Measure voltage going into brake-lever switch, volts = ???
• squeeze lever, strap to grip with zip-tie, measure voltage leaving brake-lever switch, volts = ???
• follow wire to next junction, measure voltage at terminal by back-probing connector. Volts = ???
• continue to each connector on circuit until you reach brake-bulb socket

Where power disappears indicates problem is between that location and previous one that did have power.