I had a similar issue on two separate occasions. First there was a wire that comes across the top of the transmission that vibrated around and wore off the insulation and it shorted out. This happened at 70 mph on the interstate in Florida and I coasted to the side of the road. When the roadside assistance guy showed up, in a contract truck based on a 4200 IH, he said he suspected something and went to work, finding the wire. This wire can be seen from the cab if you remove the engine cowling under the dash and look at the top of the transmission. He solved this by taking some small diameter rubber fuel line, splitting it, sliding it over the wire and taping it up. The other issue I had was with the neutral indicator wire, the tech said that there was as short somewhere between the fuse and the transmission connector, and rather than rip the whole harness apart, he bypassed it and ran a new wire taping it up to the old harness. Very frustrating both times..... Hope this helps...
LTC, USA, Retired
2008 GS6400, new to me 2014