The first step was just to test if I could transmit light through the PETG transparent 3D printer filament, and I did this in a project with my son. We made a 3D printed diesel locomotive with a string of LED Christmas lights and piped the light from the LED lamps to the appropriate lights inside the model.

I made a small block with holes for the LEDs and the fiber, 4 on one side of the block and 3 on the other side of the block. so the fibers could go to the front or the rear of the train. The fibers were then warmed with a hairdryer to make them flexible enough to position correctly through the holes in the train body, when they cool, they keep their shape making final assembly easier.

For the train, the ends of the fibers were 'polished' using toothpaste and a cloth, This brought the light intensity up by almost double. 

Although the PETG fiber is not as clear as a real glass fiber, the effect is still quite impressive. (see photos in the details section).