reverse switch and cable to vss speedo question
 

90 5.0 formula,I think someone swapped the auto for a stick.there are 3 wires coming down from the car harness and only 2 wires on the switch.well I took a jumper wire from the battery and touched each wire from the car side,2 wires nothing came on but one wire the fan and inside buzzer came on.why would there be 3 wires on the cars harness and on ly 2 on the switch.also the tranny thats in it has a cable driven speedo hook up and its suspose to have a vss.do I just have pull the cable guts out and get a vss and put it in?