I am trying to wire in an RPM activated switch to control a shift light. I have tried 2 different switches to no avail.

The box is getting power but it is not illuminating the light.

I am thinking it is the RPM signal not making it to the box, I have tapped the white wire on the black plug from the PCM. Is that correct? If not, help me out.

 

That should be the tack wire, I have always plugged that stuff into the tach output of the MSD box... We did hook our tach on the S-10 we built up to that white wire and it worked correctly.