I got the 1996/2012 A/C system working with only one minor issue remaining.
The 1996 HVAC A/C request signal from the dash module (originally) goes to the low side switch on the accumulator and then to the PCM as an A/C request signal. The PCM then grounds the relay which then powers the clutch on the compressor.
2012 Duramax has a similar thing, but of course a bit different - BCM has a 12v sourcing input that goes to the low side switch and then the dash module, when it wants A/C, it grounds the input, the BCM sees this and tells the ECM over GMLAN to energize the relay and pull in the A/C clutch.
No problem - Same idea in both systems, just inverted logic. So I run the 1996 HVAC A/C request wire to a relay which inverts the logic so it will tell 2012 "turn on the A/C, It's hot in here!". All wired up, tucked away, etc - it doesn't work
Turns out the +12v signal from the 1996 HVAC module does not have enough power behind it to actuate a 109 ohm relay coil. Drops to ~1v across the coil when activated.
Otherwise the system works great - power the coil of the relay with a jumper and the A/C turns on, low pressure switch cycles the clutch, etc. Awesome! Just need to find a solid state relay to work with the very low power signal from the HVAC module. For now I put a simple toggle switch in the dash to run the relay so I can have A/C until I find a solution.
Didn't find any green dye leaking out anywhere, so not sure where my leak is yet. Maybe the dye sealed it up