The Sniper EFI needs to see a neg (-) ground input as you noted for it's AC ON, but the control head in the dash (I'm pretty sure) outputs a 12v (+) on a green wire, which in turn signals the ECM to turn on the compressor via the compressor control relay. Have a look at the diagram posted by
69hurstolds here to see the typical diagram. Note the V8 & V6 are slightly different depending on what your car originally came with.
So, if you had the original ECM still wired up into the system, you could use the BROWN wire circuit 449 that grounds the A/C compressor control relay but since you removed all that, you're only going to have that 12v + signal from the control head when you turn on the AC.
You'll have to use a relay wired up to use that 12v + from the control head to turn on and provide the GROUND for the Sniper AC on signal. Here's a simple diagram that might work for you...