94v6a4

I am wiring up nitrous on my 95 Z28, and I cannot get the window switch to activate for anything. Everything works when I bypass the window switch. It is a MSD 8956. The tach line is tapped into the pcm. It has 8 volts at idle and the voltage drops to around 5 volts as you rev. If a put a piece of wire into the hole where the pills normally go it activates, so I know it is something with the tach signal. Does anyone have any ideas why this wont work? MSD tech support isn't that helpful.

Bandit 1

How about hooking the rpm signal wire to the negative side of the coil. I believe it would be ok with a stock ignition.