[Santa Fe Roadster]

if your lights have fog/mist inside them, one of two things happened:
1. Someone replaced your bulbs when it was humid and now the humidity is trapped inside the headlight. This can be corrected by openning the headlight and using a hairdryer to dehumidify the air inside the light.
2. you have a seal leak somewhere. Most people have had to replace the light to correct this.

If you are talking about a discoloration/hazyness on the outside of the light, there are many products that you can buy to remove the hazyness. They typically are applied in a similar method as waxing your paint.