Is the car an AOD or T5? If its an AOD, then you will need to drop the H pipe, and crossmember to get to the top of the linkage to replace. Is the speedometer bouncing around, if it is, and it is grey the it is the speedometer cable, other wise, its the shifter cable or clutch cable.
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Ryan
1990 Mustang GT (In Process)
1993 LX Coupe (Daily Driver, but just choose not to)
2005 Excursion (Big Pig)