One tail light wont turn off

Check the switch under your brake pedal. When you press the pedal the s3nsor underneath it activates the brake lights. It could be stuck down. Happened in my personal car. Had to change it out. Once it was replaced it went out.

 

If you mean that light stays on really dim
You have corrosion in the harness and it is pulling power from another circuit. Check the harness back by taillights usually ziptied to frame or crossmember. Follow wiring up you’ll find it. Pull the weather pak connection apart and willing to bet it is full of corrosion. Also unplug connection at that tail light. All connections should be pulled and di electric grease should be put in them after corrosion removed if any. keeps moisture snd salt out.