When I got my truck the 7 pin plug was missing so I went to the stealer and paid the stupid amount they wanted. When I got under the truck I found the previous owner had hacked the harness.
I installed a junction box I got at the local autoparts store, I think the mfg was DORMAN ( or something like that). It is designed for trailer wiring and is weather tight. It has terminal posts inside so you put each wire to its own post. I mounted this to my frame by the spare tire. I cut the end off the stupidly expensive harness and wired it into the box.
Oh by the way the DC wiring harness I got from the stealer was not color coded the same as my truck :--)
The junction box really worked out well because I have since gone to a goose neck. I was able to pick up another 7 pin plug and some 7 conductor trailer wire at the local trailer/RV store and wire it into the junction box. A nice clean install that is weather tight.
If I had gone to the trailer/RV store the first time I would have saved over $100. 00!
Good luck
Craig