This was a project of mine. I decided to "Open" my tail lights and leave them that way. the outer layer of the taillights is the clear soft plastic, and is simply a cover. The inside is a hard plastic trim with hard plastic red translucent lenses. I sprayed the trim black and left the lenses open for conture, and to remove the silver trim completely(the inside of the soft plastic cover is painted in a silver trim...very hard to get off).
The only issue ive had is its very difficult to find a gasket, trim price, to go between the car body and the light(or there is a gap). I eventually found a rubber trim line and put it in with black silicon. It looks ok. Im still working on it.
As for taking them apart initially. You ARE BREAKING THEM. Understand this now. There is no clean way to get them apart, and you could glue them back together, but they wont be the same. I did mine very carefully, and they came apart pretty well...still has a couple broken spots that I had to shave down.
I put them in the oven to soften the plastic, same as the headlights. you can find an open place to get a knife in on one of the corners. Just pry very carefully. It will come apart, cracks and breaks.