You must have an early car correct? If so it is heat sealed into place and you will need to cut it out. I am including some info as provided by the W113 group for you.
Also, the chrome bezel is not interchangeable between W111 and W113 despite what you may hear or read, the W111 has a curve to the top chrome bit while the W113 is straight, and will not work if switched.
Below info found at
Pagoda SL Group Technical Manual :: Electrical / TailLights
* The All-Red lights are a one piece unit with the red light unit molded to the chrome bezel. The later Red-Amber lights are two piece units. The early taillights were not designed to have replaceable lenses. Replacement was the entire one-piece chrome frame with the molded-in lens and reflector.
* The All-Red light lens can be separated from the bezel by drilling or similar technique and a new reproduction lens (or a lens from another style car) can be glued to the original bezel.
* To separate an All-Red lens from the chrome bezel (your own risk): cut the lens carefully from inside the bezel with a Dremel ® or similar small drill or grinder. Once separated, the inner "glued-in" edge of the lens will be destroyed. After re-chroming, a new lens must be re-glued to the bezel.