Rear Wiring Harness

The dealership said that I need a whole rear wiring harness, I cannot find the part anywhere or where to look for it on the truck tbh. They said it was corroded pretty bad but im thinking I should at least just try to clean it up before I pay someone to fix it or buy a new one, all help is greatly appreciated.

 

Why and what triggered the dealer visit?
There is a separate towing harness for $100 but not sure on all the tails light wiring or license plate.
Tow harness connector plugs in above rear bumper up under the bed towards the left side. It feeds the 7 pin trailer plug.

 

The tail lights are working intermentintly, as in all of them, reverse, running, brake, and lisence plate. Usually half of them work at any given time

 

For all of them to go bad the problem sounds it has a central cause. Try cleaning up your grounds. You can clean oily stuff out with contact cleaner.

 

I checked the one ground i could find but the only problem is that I cannot find a good wirng diagram anywhere, not even in the repair manual i bought. It shows what connects to what but never says where.