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.
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.