I don't have my book on me, but I know which wire you're talking about.

If it were shorted, you would be having troubles with your anti-theft, TOD (I don't know why that wire goes to the TOD though) and other systems on that circuit.

If it were open, it would have no effect on your lighting circuit, it would act as if it weren't even there.

Try to bypass the ground for the switch and relay. That's all that I can think of at the moment. Sometimes the most difficult to find electrical problems are caused by a floating/weird ground.