Here is some info from a discussion that was on a transmission shop forum last month.Vehicle was a '99 Windstar.
If it has a cable it’s either the cable or the speed sensor at the trans has stripped where the cable goes.
If it doesn’t have a cable then it’s probably the PSOM in the cluster. Part of it is functioning but the part that moves the needle doesn’t. Remember the PSOM is what converts the raw analog signal from the speed sensor into a usable digital signal for the computer systems, so if you are getting correct speed data via the computer then the VSS through the PSOM is good, it’s just the internal component that’s left. It’s all one unit built with a speedometer facing.
Another response:
The problem was missing fuse #35. I had no 12 volts to the b+ terminal in the back of IC. In the back of cluster you have a ground and 2 -12 volt terminals.
We've had a couple Taurus's that lost that power in the junction box, down under the dash. The fuse was good, but no power or intermittent power getting through. Rather than replacing the junction box, we just pulled the wire out of the box and installed an inline fuse and direct connection, to fix those.
I hope it helps.