At least one version/printing of the MPC suggests that the change from the light being mounted to the rear of the compartment to the top surface was done on Dec 1 1964. Don' t know if that reflects a specific fact but it does suggest reflection and study
For the rear mount version I don't think this is something you can order just out of a catalog. Not sure if your missing just the holder/cover and your wire is cut or more. The holder was used on other applications if I recall correctly such as cars with options that lite the ashtray and other items.
MPC shows it in the exploded view as being included with the wire loom for the whole (long style) console under the main number of 14412, which in further study shows that C4SZ-14412-B which provides you with another source (car model other than Mustang) to find a used one in good condition
Sure "wireman"
may have a comment and will post once he sees your post