Should is the important word that people who say 'do it yourself' keep using.
From my experience (more than one vehicle having a controller added) should has no part when discussing trailer wiring and connecting a truck to a trailer.
I have a very good friend who can fix just about anything. He is who installs my brake controllers for me. More often than not, when dealing with a used vehicle and a used trailer, you end up having things not work when you just plug this into that the way the instructions say it should work.
Thankfully I had an experienced person doing the work so they were able to test and determine what had been 'fixed' (aka modified) where so he could make things work correctly.
For the record, there are things in life that it is just best to hire people who know what they are doing to do. I do not fill my own cavities in my teeth or reset my own broken bones either.