one left rear brake light refuses to come on.
You would need to determine if there is power getting to the bulb socket, using a test light is handy. Check for power to the bulb socket, then work your way back to find where the power ends. There are plugins under the vehicle that cause grief because of corrosion also. Try just moving the bulb around in the bulb socket with someone pressing on the brake pedal to try and may contact. May also be a bad bulb socket or ground wire.
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