It looks like your rear light has only one wire going to it. That would be the hot wire. But it also needs a ground. I would guess that it is designed to ground through the frame. That may be what the little threaded hole on the clamp is for, to accommodate a pointed screw that would pierce through the paint to the metal stay to provide a ground. With the cloth tape and paint blocking the ground path the light won't work.

You will need to find a way to ground the light. An easy solution would be to run a wire from the mounting clamp of the light to the mounting screw of the generator. You can test my theory by temporarily attaching a wire between those two points and then testing the light.
Brent