I,m not sure on the Ford Taurus,but most all cars dropped the cable and use a sensor to relay the transmission out put speed.Most are located on the transaxle assy,on the passenger side of the vehicle or off center in that direction.The sensor has a 2 wire hook up, and inside the housing uses a reluctor coil and a spoked wheel,that makes a signal or pulse, like the solid state ign system for the eng.This goes through the circuit to the computer moduale and then to the speedometer head.
I don't know this car specifically, but there's probably a speedometer cable and a sensor. The problem could be at either end--with the sensor or with the connection at the dash.
I recommend getting a repair manual for this car and checking the diagrams--they probably have a manual at your local library. That'll tell you whether or not you can replace the part yourself--could be a big job.
Your 93 ford taurus has both a speed sensor used for your cruise control and antilock brakes system and it has a speedometer cable that operates the speedometer.If tyhe speedometer is not operating corectly the cable is suspect as is the speedometer head itself.