I would bet that either the pto safety switch or the clutch switch is the problem. could be a corroded terminal in one of the connectors, or the switch itself. The small terminal that doesn't have voltage will be wired through the safety switches to complete the start circuit.
Take a ohm meter on connect the red lead to the wire without power, make sure you don't have power first, and the other lead to ground. If you don't have continuity that is were the problem is someplace in the wire circuit.
Take a ohm meter on connect the red lead to the wire without power, make sure you don't have power first, and the other lead to ground. If you don't have continuity that is were the problem is someplace in the wire circuit.