11. Now preheat your oven to 200 degrees Fahrenheit. Place the headlight housing on a baking sheet and put it in the oven for about 10-15 minutes. Seems like most people don't have problems with 15 minutes, but some needed as much as 20. Just do no more than 15 to be safe, try to pry it apart, and if the sealer isn't melty enough yet, plop 'er back in for a few.