From my 67 shop manual:
Taillight body - Mustang
1. Remove the spare wheel and tire assembly.
2. Unplug the bulb and socket from the light body.
3. Remove the six plastic covers from the light body and bezel (doors) retaining studs.
4. Remove the six nuts from the light body and bezel (doors) retaining studs.
5. Pull the light body back and off the studs. The light bezel (doors) can be replaced if necessary at this time as the studs are on the bezel and these studs and nuts retain the light assembly to the body panel.
6. Remove the gasket seal from the front of the lens.
7. Remove the six lens retiaing screws from the light body and remove the lens from the light body.
That is it for removal. Assembly is basically the reverse.