I did this before on a friends ranger- don't remember which method.
One method:
Preheat your cooking oven @ 250 degree for 5 minutes.
5 min is up, sit a piece of WET cardboard on top of the plate.
Place your headlight on top of the WET cardboard. (make sure no wires or bulbs are attached
Bake your headlight for 7 minutes.
After 7 minutes, remove it from the oven. The silicone should be melted. With your bare hand, try to open it up by tugging and pulling. If you are not the Incredible Hulk like me, you may need a 2nd person to help you or use a flat head to pry it open.
While opening, make sure no silicone stick inside your housing. If yes try to clean it off.
In case you can't open it. re-wet the cardboard and throw it back in for about 3 minute should be good.
Another:
Once you have stripped the headlight housings, it's time to bake them in the oven so that the lens can be easily removed. They must be done one at a time. Place each headlight on a cookie sheet with foil or a peice of cardboard to prevent the headlight from melting against the cookie sheet.
As far as temperature, I found that 305 degrees for about 9 minutes works well. Remember, preheat the oven and then once you put the light in there, turn the oven off. While baking try to avoid opening the oven to check on the light. This will prolong the heating process. At this time and temperature, the lens should pry apart from the housing on the first try. Just take your time and don't rush anything. This is not something that just peels apart super easy. I didn't use any tools. I just used my hands and started in a corner where its easy to grab both parts. I also had a buddy helping me pry it apart slowly.