Larry,
I believe your last reply gave a big clue to the problem. GM, sometime near the time period of your "old" brake controller harness that you kept from your old vehicle, changed the wiring and the old harness will not work with newer vehicles. I would bet a beer that if you get a new harness, specific to your present vehicle, that your problem will be solved. Just about everything you have said points to a wiring problem in my opinion. It might be best to just do as I did and hardwire it in and forget about the harness.
Barney
