The control signal wire for the Solenoid comes from the Starter/Headlight-Cut relay which is the 5-pin one behind the right side cover (the other on that side is the 3-pin flasher relay)
Check the headlights - do they go out when you press the start switch?
The starter/headlight-cut relay sends power to the headlights via the normally-closed terminal when it is NOT energized;
when it IS energized, it BREAKS power from the Headlights and sends it to the Starter Solenoid.
Common issue that interlocks the starter relay is the clutch switch is not closing.