I'll add what I do know for the benefit of others looking into the same problem.

The Chevy/GMC trucks that came with a towing package have the prewired plug ONLY IF IT IS THE MODEL AFTER THE LAST REDESIGN.

For TRUCKS, that is any model year 1999 or later. For SUVs, it is any model year 2000 or later.

So, if you have wired a brake controller in a pre 1999 truck or pre 2000 suv and have any helpful hints, I would appreciate them.

PS I did check with my GMC dealer and was told, "No, sorry, no harness or easy connection."