Peterbilt starting issues

I have a 2004 Peterbilt 379 that has started giving problems starting. Occasionlly I will go to start it and it will not do anything. There is a relay on the firewall that when it starts will have a wire in it that is continuouslly hot but when it has a spell and don't start this wire is not hot. I traced it down to a connection on the firewall and the hot wire in that plug is a black wire with an orange stripe. I am hoping someone will know where this wire is coming from or what may be the problem. The black wire with orange stripe appears to go under the cab instead of in the harness that plugs in the firewall. Thanks for yall's help.