Peterbilt won't start

I have a 2001 Peterbilt 379 with a cat c-12 engine. All circuit breakers are pushed in so its not that. Also changed the box that's on the firewall. But when I turn my key all lights come on and then go out. When I try to start it does nothing at all. But if I take the female plug that goes in to the relay box on the firewall out and run a wire across it the truck starts perfectly which rules out the starter or batteries being bad. I'm guessing it's the ignition but I'm just trying to get some input

 

Thanks I will try that tomorrow and see what happens. Appreciate the response

 

No it hasn't had any leaks at all that I know of. When I turn the key on everything powers up the way it's suppose to. But when I try to crank it it does nothing at all. No clicks, no grinding or anything. But if I unplug the relay on the firewall and jump a wire across the female plug with the key on it'll start right up.