alright, a quick electrical lesson.
There are 4 major components in the lighting system. The lighting coil (which generates between 7 and 35 VAC), the voltage regulator, the handle bar switch, and the light bulb itself.
The lighting coil has a resistance spec listed in the service manual (read as Ohms) relative to chassis ground. I believe it's the yellow with red striped wire coming out of the front left corner of the motor and connecting to the wiring harness just inside the frame on the left side (about where your leg sits).
If it is reading the correct resistance, you can start the motor and check for VAC. At an idle it should be putting out ~7VAC.
Once it goes into the harness it actually splits. One "leg" goes to the voltage regulator which acts as a "dead short" for all voltage over ~14VAC to limit the voltage on the lighting line to that.
The other "leg" goes to the handlebar switch where it is switched between the outgoing yellow and green wires (high and low). From there it goes directly into the lightbulb and then the lightbulb is connected back to harness ground.