For the 1967 Coupe, 1968 Coupe (early production through mid year) and all 1967 through 1969 Convertibles, the power for the trunk light came from the license lamp, so the lights (parking or headlamp) had to be on for the trunk light to function.
For the late 1968 Coupes and all 1969 Coupes, the power for the trunk light came from the dome light harness, so it was hot at all times and all you had to do was open the trunk lid.
Note that the trunk light was adjusted at the factory so it would come on when the trunk was opened approx 12" (plus or minus 3") when the car was on level ground.
Ed