My tail lights dont work. My brake lights &blinkers all work. Not while driving

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While driving I have no tail lights. Brake lights and hazards and blinkers work fine. Changes bulbs checked connections in lenses checked fuses. I have brought the car to 5 different shops and no one can even tell me what the problem is. I get pulled over 2-3x a week. Between tix and estimates for diagnostics I have spent over $1000 and have no idea what the problem is. I just don't get it. Someone please help me!! Thanks for any ideas. Again shops and me have done all possible routine checks.