1996 Cavalier, Tach, AC Input?

Trying to get tach (AC) signal for remote starter. Book says I can use negative side of injector as a reference. Which color is the negative? The common grey to all injectors, or the colored wire? I tried the PURPLE / white at ECM but remote starter did not pick up the signal.

I had it on the colored and it did sense the signal. But the car would not run it would fire and then stall/shut down. So I though maybe the tach signal was bad? 95% sure on all other wiring. I will have to check everything again I suppose.

Thanks for any help/guidance.