Under Dash Courtesy Lights for a '66

Are you sure they are the right size? What happens when you put them in? Are they not going in far enough for you to twist and lock them in place? If so, try pulling up on the wire. The sockets are spring loaded, so if you pull on wire then you are compressing the spring making it easier to get the bulb in.

The bulbs light up but will not click in. I was given to them by a guy in a Mustang shop. Where is the wire located that compresses the spring?

On the back of the socket