I've never seen a schematic where the seat switch had anything to do with the crank circuit.

Have you done the most basic step and check the fuse?

Not yet. Not sure where it is.

Seriously- IF you can't find the fuse, do you think you'll be able to fix anything else with instructions over the internet?

Nice, very nice. You don't know anything about me. Feel free to ignore this thread from now on.

Deere wiring diagrams don't actually tell where to look for an over-current device, at least not in words.
And looking at a few STX diagrams online I see that Deere may even have used (on various production years) a fuse, or a circuit breaker, or even a fusible link...........to accomplish the same protection for this circuit.
I can tell you in electrical terms where you will need to begin searching for whichever type of interrupter yours has, but you will still need to physically do the hunting.
The fuse, circuit breaker, or the fusible link will be part of a circuit coming off of the BATTERY CABLE POST on the STARTER SOLENOID (not the solenoid starter cable post) .
The circuit will be a small wire that is on the same post of the starter solenoid as the cable coming from the battery + post.
If you can locate that wire and trace it you should find whatever your model uses.
The circuit originates at the starter solenoid and ends at the key switch, and the circuit is in fact the hot feed for the key switch.

OK. I can do that. Thank you for taking the time to answer. I'm not being too cheap to take it in for service. I simply cannot afford to right now - just not an option.

Based on your serial number, there is a fusible link between the solenoid and the starter switch. I think that the wire from the starter switch will be red. I don't remember if the fusible link is in the box that shields the starter & PTO switch, or if it is nearer the solenoid, but if the circuit tests okay it won't matter.

You mentioned the "hot" cable off the battery, but did you check the ground cable at both ends?

Yes,I checked the ground cable from the battery to where it bolts to the frame. Looked intact and free of corrosion.

Thanks also for the other info. I'll check inside the starter box if I can't find the link along the line.

What exactly are you calling a "starter box"?
My reference to Starter Solenoid does not imply that you will be dealing with the STARTER itself. So forget about STARTER for now, we will get to that if necessary but not now.
At this point we are only trying to confirm whether there is in fact a good circuit feeding the key switch. That circuit begins at the starter solenoid battery positive cable post.
I might as well ask: Do you have a 12 volt test light? You will need a 12 volt automotive test light, or a volt meter, to accurately test circuits to solve these kinds of problems.

Sorry. I was talking about the box that covers the key switch. I have a volt meter, but it's so old that I don't trust it. I'm borrowing one from a friend today.

Had the same problem. tired new battery,switch, but it ended up being the fuse able link. it starts at the hot side of the starting solenoid as a black wire then into a covered link changing into a red wire that goes up to the switch. I removed the wire and when I cut off the heat shrink the link was corroded and fell in half. I replaced it link with a simple in-line fuse. It started right up and has been good ever since!