Have someone turn on the signal and step on the brakes.
With the tail light still installed in the socket:
Touch the unlit bulb with one end of the wire. Touch the metal bulb base right where it meets the glass.
Touch the other end of the wire to a known good ground.
See if that make the light come on.
With the tail light still installed in the socket:
Touch the unlit bulb with one end of the wire. Touch the metal bulb base right where it meets the glass.
Touch the other end of the wire to a known good ground.
See if that make the light come on.