If the fog lights of the car are broken then it is not safe and it needs to be repaired. Its work is to indicate the activity on the road, which is beneficial for your car as well as the car behind or ahead. They are also given direction guide. Therefore, if the fog lights are broken then it should not be done. And if it is not used then it is not considered legal. So, you should understand that not it is legal to drive with the rear fog light smashed.