Tach signal question

I want to install a MSD RPM window switch for a NOS kit I installed on a 99 SV. Does anyone know where I can pick-up a tach signal for the window switch? MSD recommend their tack driver part number 8939 and to connect it to the positive side of one the coils. We tried the coil connection and connecting to the ECU tack wire but neither worked. Another shop recommended using an Autometer tach driver part number 9117 but it appears to wire the same so I have not tried it as of yet. Any information and help would be appreciated.

Thanks, Greg