Re: 65 Galaxie bakseat courtesy lights
If you're talking about the round snap in courtesy lights on the regular galaxie 500 then the whole thing snaps in the sail panel. Problem will be after decades the sail panel may become brittle and trying to remove the lamp housings will damage the panel.
On the one end of the round chrome bezel there should be a little notch intended to get a tool under the bezel and that part should pull outward, the socket and wire are on the other end so once it's unclipped it needs to come out on an angle.
Change the bulb and installation is the reverse.
Hope that helps.