Last weekend, I went out to start my tractor and nothing, not even a click. Check the battery and it's fully charged. That tells me it's the safety circuit again. I knew I would have problems with this tractor from the day I bought it since you could tell the previous owner had modified the wiring already, so I did a complete rewire. Added some relays so I don't have to use the old pull switches if I don't want to and simplified things. It works great now, and I even got the lights to work that didn't before. From top to bottom the relays are Kill Switch, Lights, Front PTO, Rear PTO. I have room for more if I ever find a cab and want more lights on a different switch or something like that.
The reason it didn't start before the rewire was because the rear PTO switch had fallen apart. I took it out, found the pieces, put it back together and peened the sides back in. Works like new.
I am an Industrial Electrician by trade, so this stuff was actually really fun to me. The relays aren't necessary, you can do a complete rewire without them, but I like having them. Plus they are able to be mechanically turned on by hand, so if something is wrong they actually help with troubleshooting.