check the upper left hand side of the transmission. there is a little metal clamp that screws in to hold the speedo cable in place.
My speedo had quit working because the screw that holds it in place rattled out. It might just be as simple as tightening or replacing that.
That bieng said, if that part looks good, Check behind the cluster.
I would almost guarantee that's what it is . . . It's very common with Humvee's to sheer off the retaining bolt or yank the speedo cable but haven't had it happen on a Stang . . . Do much off roading ?? Even if it is still in tact, I would go one step further and check the gear itself. It's as simple as loosening the one retainer screw and the cable/gear assembly slides right out. It is just a plastic gear, though not likely, it could be worn or failed in some way . . . only a $15 part, not a real big deal and easily repairable . . . especially as opposed to the next step which would be to check the other end of the cable and the speedo itself. This means removing the gauge pod itself from the dash . . . though not really difficult, quite time consuming. Take care in removing the speedo cable from the rear of the gauge pod once you get to it, it's a real tight reach. Once out, try manually turning the speedo ( have to do it really fast ) if the needle still doesn't budge you found your problem. It's the speedo itself. If it does move, it may be the end of the speedo cable or the inside where the speedo cable inserts into the gauge pod. Another helpful hint to this before you do anything is to check your odometer to see if it is tracking your mileage . . . if so, the speedo cable is most likely fine. It's your speedo pod.