I've been having some trouble with this as well. Not sure if you are using HPTuners or some other platform, but mine will only use the brake signal (light blue wire going to ECM and TCM) when the engine is not running.
I'll update when I get a chance to work on it, but as far as I can tell this signal will originate in one of two ways:
1. Either the light blue wire also goes to the BCM and it has to send out a signal (unlikely)
or
2. We need to use the brake position sensor. If you happened to follow the E38 pinout on
LT1swap.com, you will notice that pins 26 (signal), 42 (low reference), and 54 (supply voltage) in connector J1 are "Not used". This is where the brake position sensor should be wired in. I went ahead and ordered one, but I won't have it until next week.
I've been struggling to find exactly how the programming in the ECM is setup. It is unclear how the brake position switch (3 wire), and the brake switch (1 wire) work. Some reports indicate that a faulty brake position switch could make it hard to impossible to shift from park, faulty brake lights, and the cruise control system not working. But it doesn't sound like these always happen with one another.
I did find that you need to relearn the pedal park status in the ECM/BCM so you will need a GM tech2 scanner. For now I'll just actuate it in my hand to see if it triggers the brakes applied signal without the BCM present. I'll update next weekend.