For reasons unclear to us, Ford didn't provide an overhead dome light in '65-'66 Mustang hardtops-only small lamps for the foot area. Yet, there's a provision for a dome light hiding behind the headliner in every '65-'66 Mustang hardtop, which means you can easily install one yourself.
This is the Scott Drake dome...
This is the Scott Drake dome lamp assembly from California Mustang (PN C0AF-13A719-B). Drake took the Mustang's original dome lamp one better with a large plastic reflector to yield more light. You can use the wire splice included in the kit or you can plug in behind the instrument panel.
There are two approaches to dome light installation. The first is to find a '67-'70 Mustang hardtop parts car and use its dome light, which is nothing more than a socket/reflector dish combo with a long wire and male plug. The second is to order a new Scott Drake dome lamp assembly kit (PN C0AF-13A719-B) from California Mustang. Whether you're installing a factory original dome light or the Scott Drake kit, the approach is the same.
As part of my search for a "victim" for this installation, I posted a note at the Vintage Mustang Forum (www.vintage-mustang.com) and heard back from Jim Williamson, who lives about 40 miles from my doorstep in Southern California. Jim graciously offered his daughter's good-looking '66 Mustang hardtop.
 To get power to the dome light,...  To get power to the dome light, pull the rear seat and lefthand quarter-trim panel. To pull the seat, push in at the bottom and pull up. The seatback also needs to come out. Simply remove the two sheetmetal screws. |  The quarter-trim panel is...  The quarter-trim panel is retained with five sheetmetal trim screws. An Allen set screw retains the window handle, unless you're dealing with an early '65 (before March 8, 1965) with clip-style handles. |  Jim Williamson didn't have...  Jim Williamson didn't have a scratch awl handy, so he used a test light, which is just as effective, to find the factory dome light screw holes. You can feel them through the headliner. Use a razor blade to cut a 2-inch slit to gain wire access. Be careful not to tear the headliner. |