The start solenoid blocks the flow of current to the starter unless it is triggered. There is a starter solenoid on the starter itself.
[on edit]
I went back to the beginning of this thread. When you say that the coach won't start from the front, it looks like you may be saying it will not even try to engage the starter if the Starter switch in the rear is OFF and the Safety is ON. Yet you indicate that from the rear, the Start lever will engage the starter but not start the engine without the key being on. Although that is not the way mine works, it is possible that yours is wired where the key would need to be on to actually start the engine from the rear.
If these statements are correct, that would mean that the rear switches are working, potentially even properly, BUT the key is not able to cause current to flow to the starter when the Safety is ON and the Starter switch is OFF. If this is the case, the problem is likely that the wire from the ignition is not activating the Start solenoid. If the Safety is ON and the Starter switch is OFF and the key will cause the starter to turn over the engine but the engine will not actually start, then the problem is likely that the key isn't feeding power to the fuel solenoid as others have suggested.
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