Here's a link all about headlights and HIDs:
Also the direct instructions from the page on removing lense and cleaning headlights:
How do I remove the front plastic glass from headlight?
OEM healights usually have a seal that will soften with heat. The main idea here is to soften the seal, but not heat damaging headlight. This might not work on all headlamps. Do not use an iron, butane torch nor heat gun. It will garuanteed warp plastic. An added downside with warping is that material gets incrased tension inside, and will crack easier when it gets cold. (Herman have tried).
The best is to use an oven. Preheat oven until reaching 400 degrees F. Then TURN OFF oven. Then insert headlight so that it doesnt get in direct contact with anything, by using a few layers of holy tinfoil or cerimc dish, etc. Wait 10 mins. Remove product. Put on hot or welding gloves. Pry carefully with screwdriver or even better, a butter knife, so lense comes off of reflector housing gradually, working your way around seams.
10 minutes and 400F are general numbers, and will need to be adjusted depending on sealant used in headlight.
How do I clean a reflector lamp?
Cleaning the outside of a headlight should preferrably be done with soap and water, and is considered very straight forward.
However, cleaning the inside of a headlamp becomes more tricky since you will face the same challenges as cleaning the inside of a bottle:
Remove the headlight bulb.
Remove the headlight from the car.
Fill the headlight about 1/2 full of distilled water and put in a few drops of dishwashing liquid.
Cover the bulb opening and shake the housing for a minute or so.
Pour out the soapy water.
Fill the headlight about 1/2 full of distilled water.
Rinse again with 1/4 cup of rubbing alcohol added (promotes drying).
Let headlight dry, open end down (overnight), or you can use a hair dryer to speed things up.
Not sure if this works....