Well, if your trailer is set up like my 73, the casting is held to the shell by pop rivets that are installed from the inside side of the skin. If you are very lucky, there will be a plastic access cover on the inside that might give you access to the rivets as well as the backside of the wiring going into the housing. I am assuming you haven't removed all your interior skins.
If you do not want to remove the castings, and only mess with the light assemblies themselves, then start by removing the colored lens, and you will see a rusty metal cup shaped device refered to as a "can." The can holds the bulb and is pop riveted to the casting.