Sensor that says door is open doesn't go off when all doors shut on 96 mercury sable need advice.?

Sometimes when I get into my car and start it up, then drive my light in my car doesn't go off and the sensor on the dashboard says that the doors are still open but I know all the doors are shut even after slamming them shut it still says it is open. What could be wrong? I also had a time where my friend was in the pengers seat and her door flew open when I turned the corner.