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.

 

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