Update - success!
I thought I'd make this last post and update you on my success. As of late this evening, I have a running and moving 455D with a properly working seat safety system. If I am not on the seat and engage the PTO, it dies. If I am not on the seat and try to move the unit, it dies. When on the seat, everything works as normal.
I started out yesterday by having one of the electricians that I work with test the switch and it was/is OK. I told him of the problem I was having and what the safety circuit consisted of and he suggested going right to the relay. After work today, I removed the relay and took it apart and cleaned the contacts. I put it back in the system and no improvement. I then jammed the relay in the closed position and re-installed it and then the traction unit would move as normal without shutting off. Only problem was the PTO would not work. I took the relay back off and removed the shim so it was functional again and re-installed only to find that it was back to stalling when the pedal was depressed and still no PTO. I then went inside the control box and was looking for corrosion when I noticed a burned out 5 amp fuse while checking them. I replaced the fuse, started the unit and what do you know, everything workd as it is supposed to.
I am confused as to what actually happened since the only real thing that was done was clean the relay contacts and replace the dead fuse. I did have to adjust the seat switch to make it taller as it was not making contact with the seat, a fact that never mattered before as the seat circuit was not working. I still have no idea how or what made it possible to use the traction unit for 5 years with the seat safety system offline. I found no jumpers or any evidence of tampering with the wiring. All I know now is the system works as designed and it only cost me a 20¢ fuse.
Thanks for the info, it got me thinking and pointed in the right direction.