Probably something like this...
http://www.nicoclub.com/articles.php?id=267092
I wouldn't recomend cutting the plastic though. Try putting it in the oven for a while to break the glue loose.
Idk why you would want to paint the inside of the taillight though... probably wouldn't make much of a difference because the plastic is red.. so it should output like a dark red if you paint the inside black..?