Just follow the cable sheath back to the
tranny from where it comes out of the firewall. There should be one small bolt holding it into the
tranny. Remove the bolt and pull the sheath and cable, (the cable is in the sheath), from the
tranny. There should be a little plastic gear attached to the end of the cable. If there isn't, then that is why your speedo isn't working. You will have to get the gear out of the
tranny, if it hasnt desintagrated and replace it. You can get the cable from the parts store by asking for a speedometer cable for your year car. Not sure if the parts store has the gears or not. Ask. If not, there are plenty of places like NPD and Mustangs Unlimited that have them. Don't forget the little clip that holds the gear on the end of the cable. Before you replace the cable, make sure you grease it or it will make a racket beating on the inside of the sheath. I would also reccomend getting a Haynes manual from the parts store for your car. It will tell you how to replace the cable and show you pics.
__________________
1966 Mustang Coupe. Mica Merlot with black stripes, Deluxe interior, 302 4V, 4-speed toploader, 3:70 9" rear end.
1973 speedometer cable NOT turning 
