You're interrupting power to the PCM brake switch input? Light blue/black wire on pin C14. When I did my LT1 transplant I just put power to that wire. Works fine unless you're stepping on the brake while holding the throttle since the PCM is programmed to unlock the TCC at 0% TPS.
I believe A12 is a PWM output to the EBTCM to tell it what torque or throttle percentage or something similar that the engine is producing. Pin A23 is a PWM signal back to tell the PCM how much to reduce the engine torque, by retarding the timing.