I had the lights split open on my S15 for light conversion. Didn't do it myself though but on my Skyline i did the work myself.
Couldn't fit them in the oven so I baked them in a drying locker for clothes. can't remember the exact heat, but I think it was around 60 degrees for 20-25min. could have been warmer but it was below 100 degrees C. Then I pulled the parts apart without any use of tools. remember that the plastic gets pretty soft when hot so don't just use screwdrivers or something like that to pry it open.
People have used heat guns to, but I think an oven or drying locker is better to get the heat more evenly distributed.
Just think about not overheating the light so they melt and you'll be fine
Although I would try to use T-cut or a good laquer claener before attempting to open them up.