Hi...I wish I could answer on the harness. Mine was hand built by James & Lenny at Rich's tech show. Then they installed and tested so no problems.
James explained to me being hand built cars each car can be slightly different.
On my car they had to modify the harness to make the cameras work. They stated maybe one in ten units does this to them.
I would believe they are like you stated currently using off the shelve harnesses. Rich sells them and he is an honest guy and tell you if you asked him.
All I know is I almost ruined the front of the car parking or not realizing how close to things before I had installed.
Again...great job understanding and repairing your car.

 

New GpS module arrived all working now

 

Success! Congrats on resolving! It's a great feeling figuring these things out on your own. Especially when avoiding the dealer shop rates!