No tail lights

What is your question?

plugged in a trailer light wiring harness and connected the ground wire to the metal body of the car. I now have NO TAIL LIGHTS on the car and likewise no tail lights on the trailer. However, the blinkers and brakes lights do work on both. SO...I removed the wiring harness and the tail lights on the car still refuse to work. Is there a daylight sensor where I need to test this at night??