The seat switch is disconnected. I don't know where the others are or how to test them.

Wayne,

You should have battery voltage at the main starter post. That's a constant hot straight from the main battery post.

You should have a signal with battery voltage active on the small wire of the starter solenoid with the PTO off, brake engaged, and keyswitch in start.

Your ground lead for these tests should be hooked to the engine block ground just in front of the starter.

Since it will crank when you jump the solenoid, I suspect you have an issue with the signal wire. To test the solenoid, you can jump from the main starter post B+ to the small tab at the solenoid. If it behaves normally, you have a control side issue. I would start looking at your brake safety interlock and go from there.