You need to work through and test each component.
- Make sure the starter motor pinion isn't jammed up in the flywheel. Disconnect the battery cable and starter motor cable from the solenoid. Measure the resistance between the starter motor cable and ground.
- Disconnect the connector in the orange wire leading to the solenoid. Measure the resistance across the battery and starter motor terminals on the solenoid.
- Measure the resistance between the orange cable connected to the solenoid and ground.