Is it a MTX or ATX?
on my 94 ATX it runs from the back of the cluster to a joint near the brake cylinder. From there is goes behind the engine to the VSS. The VSS plugs into the governor assembly which bolts onto the tranny.
Easy check is to remove the two cables from one another and spin the upper cable while someone inside checks for playback from the speedo. Should spin in the counter-clockwise direction(if im wrong please correct me). Then spin the other side, if it doesnt spin then your good. You just cancelled out the cables, the cluster, and the gear on the VSS. Now you can pull the Governor (IF SO EQUIPPED) it has two 8mm (5/16") bolts. Has a PIA cover to remove. Easiest to go from the top with the intake off, but you can also go from the bottom.
Remove it. There should be two gears mounted on a shaft and the cover which comes out. The smaller gear should be straight. If it is in any way concaved, If you were to look at it sideways the gear should look like a rectangle. If the sides are concaved you solved your problem. That part costs about $45-50 from the dealer. Worst case scenario is the gear in the tranny and That bad boy if done at the factory or rebuilder would set u back about $450 Easy.
Good Luck. If I can find photos I will post them. And again if its a MTX I dunno exactly if the above mentioned checks would be the same. Someone else might be able to help.
Maybe cause I spun it a little too fast messing with that mustang yesterday. 
