Tachometer not working.

[Burto]

I have a '73 that I put together from parts. It runs great but I too am trying to get the tach working. I upgraded to the Pertronix ignition. I understand where to connect a gnd and the +12volt wire, but I am confused as to how to hook up the "current loop" I bought a spare tach, was told it was working and was planning to "test wire" it under the hood before pulling the old one out of the dash. One side of the current loop has a piece of red tape and the other side has a piece of black tape. Where do I connect the other two wire?