Need to recalibrate my digital speedometer on my 87 Firebird Trans Am. According to the wiring diagrams, the VSS is wired into a VSS buffer under the dash. It looks like the buffer splits the signal, and sends a 4000ppm signal to the digital speedometer and a 2000ppm signal to the PCM.

If I'm thinking correctly, the dakota box would have to be wired in between the VSS and the buffer. Am I correct? Otherwise, if it were wired in between the buffer and the speedometer, you'd have the correct speed on the speedo, but the wrong signal would be sent to the PCM. Am I correct with my train of thought?

Also, does anybody know which wire from the VSS is the signal and which is the ground? The diagrams show the VSS has one purple and one yellow wire.

Thanks guys!