Lights dont work--NEED HELP

my 99 sonoma drivers side taillight wont turn on. the bulb is a 3 way(taillight, turn signal, brake light) and the turn signal and brake light work. All my fuses are good, and the power to the bulb looks good. obviously the light is getting power. ive replaced the bulb 3 times. anyone got any ideas???

id check to see if a wire is broke

we checked all wires could it be the thing the bulb plugs into? maybe that got burnt out? the ground is fine and so is the box all the wires go into, any other ideas. thanks

On my 98 the taillights have independent power sources. They are the brown wires. Mine melted to the exhaust under the cab.