1955 Cub Rear Light

I am finally getting around to finishing up my 1955 Cub. The wiring harness I have has two wires running back to the rear light. One is green and one is black. Which
one do I connect to the light---assuming I connected the right one to the switch up front? It is a six volt positive ground tractor. What is the second wire for?
Thanks in advance.

According to the wiring diagram, the black is to the tail light.
Do not know about the second wire.
What is it attached to on the other end ?

One light should be power to the rear light and the other should be a ground for the light. I think the power wire should be black and the ground pink. But maybe the pink was just used on later tractors. The front end of the ground wire should have a large ring connector that can fit a bolt that holds the instrument panel to the torque tube. The power wire will have a small loop that fits a lug of the light switch.

With the ends apart and not touching the tractor use a meter to find the hot one or look at the other end hot will go to the light and the other will be ground unless you happen to have the red light mounted on the rear light then both could be hot to light the red one. Don't think those were on all tractors nor on the Cub.

Thanks to everyone. In this case, There is a black and green wire. I checked and I had used the green wire as the ground wire near the instrument panel and the black
wire is wired to the switch. At the light end, both ends are the same. I will wire the black one to the light and try to find a suitable ground for the green one. I
might have to replace the loop at that end to fit a bolt. Done this before on other tractors, and getting a good ground can be a bit tricky with all the paint, etc.
Sometimes the manufacturer expects the light to be grounded just by being fastened on to something, but sometimes I have just had to run a separate ground wire from a
good ground to the light. Probably originally there was not a ground wire, but this is an aftermarket wiring harness and the maker probably just put in a ground wire,
which is a good idea. I just was wondering if I was missing something. I hope to put a battery in the Cub today and see what happens, and then get her started for the
first time in a couple of years. I have turned her over with the crank occasionally just to make sure she does not stick. Thanks again to everyone.

If you have the original style lamp, it has a screw that grounds the bulb to the lamp housing, just below center on the back side. That should be a good point to attach the ground wire. I don't know if that was the original point, but assume so.