How to install rear tail light trim?

[Hawssie]

I pulled out some of my new old stock parts from my stash to install on one of my trucks. A previous owner filled in all the trim holes already. Anyways I have installed most of the trim but the pieces that go around the tail lights are hanging me up. On the inside side there are 2 screw holes but the outside and top there are some sort of retainer tabs. The only way I can figure out these go on is that the body must have slits cut into it? Is that right?