My wife's 2005 Grand Cherokee left tail lights don't light when the head lamps are on - right side works fine. All 3 bulbs are good - checked bottom reverse light - OK, and even replaced middle bulb. Middle bulb works only when blinker turned on. Top bulb works when brakes on, but not when headlamps turned on.

 

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Check for melted plastic in the bulb holder or a bad ground connection.