A number of years ago, I installed the third light on 3 of our family's 65-66 Mustangs. The installation was simple and the light mounted on the rear package shelf. The diode logic module that powered the unit was about 2x2x1 and went in the trunk. The center light only came on when both rear brake lights came on. Thus, it did not illuminate with the turn signal. Seemed to work OK and minimized the tailgating when coming up to a stop.
I robbed a 1998 or so S10 Blazer of it's roof-mounted third light. What caught my eye is that it's slim, about a foot long. and most importantly, black when the light is off.
I haven't mounted it but have trial fit it on my '67 coupe. When I put it on it will be on the rear package tray just peeping over the rear window weatherstrip. I looked at screing it to the underside of the roof the same way which hid it even better but I could see it a bit too well in my rear view mirror. I haven't completely decided yet though. Either way looks to be pretty stealthy. '66's having a different back glass area might make this particular light fit better or worse, I couldn't say.
I really prefer the 3 bulb incandescent setup. If you do go with LED tail lights, get the Mustang Project ones. The look so much better than the others since they have more smaller LEDs and you keep your lenses.