its caused by condensation, i had this problem and what i did was took the headlights off and disassembled them and cleaned out the lens and then reassembled the headlight and sealed it on top with sillcone so no air would leak in and put the headlights back on and problem solved, the headlights look ghetto with the sillicone, but its a $10 fix, and you dont even see the glue unless your hood is open and its not even that bad, that or buy new headlights, ive never tried sanding like the other ppl are saying