I need to know which wire is the ground wire from the 700r4 vss. Also, would anybody happen to know which wire is the ground from the t-56 vss? Thank you

They're not the same?

Since I swapped to a T-56, I need to use the SGI-5 to recalibrate my speedo. I need to splice into the old line to do this-this is why I need to know which wire is the ground.

It's an AC signal, so just pick one, any one, and ground it.