Using a multi-meter to check continuity of switches in their various positions. You're right, of course, there could be a wiring issue. I'm searching high and low trying to locate some sort of wiring diagram for the tractor. No luck so far. Would make it easier if i could just follow the wiring and switches in order and check for voltage at each stage.
To answer a previous question, there appears to be a starter relay mounted on the firewall. My wife found it by pulling hard on the bottom of it where the wires go in (I had NO idea that she was doing this.). Well, it popped out of it's housing. Nice bright shiny copper assembly that makes really big sparks when you let it touch the chassis. VERY carefully inserted it back into its housing, and told her to NEVER do anything like that again. Fortunately, it didn't seem enough to damage anything.
Now to find some sort of wiring diagram for the tractor.