auxiliary trailer harness on 379

I'm trying to hook up the dump valve on my spread. I am trying to use the auxiliary harness that the truck came with. After chasing it up to the firewall, I noticed that three of the wires are stubbed off, and do not go into the firewall cluster. I put power to one of the wires, but it did not show power at the plug behind the sleeper. I could not find anywhere where there may be a break in the wiring. Can anyone help me figure out what to do? I hate to see the harness just go to waste. If I can figure this out, I have plans to hook up other stuff on my trailer, to be controlled from the cab. Thanks. CW

 

I did look at the supermiller schematics, but couldn't make heads or tails out of it. As it turns out, the stubbed out wires were not connected in the plug behind the sleeper. So, a few bucks and a new plug, and everything is working great. I appreciate the input, though.