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The new harness will have a vss that goes from the trans to the pcm. The pcm has a speedo output that runs to the old speedometer.
I, on the other hand, spliced into the new vss and connected the old vss to make my speedo work. The way I did it is not how I recommend it and I will probably change it in the future. If you use the speedo output from the pcm then it is tunable for gear changes, tire sizes etc.