Speedo calibration using "yellow box"

I posted in H-T asking how do I change my speedo's calibration. They referred me to this site, Black Robotics, and that was pretty much all the help they would give me.

I am going to get this yellow box, which simply splices into the speedo signal wire. My problem is that I am unsure how to find the speedo signal wire. My Chilton's is absolutely no help, and I haven't found anything about it on the internet.

How do I figure out which one carries the signal?