[68gmsee]

If the needle doesn't move at all I would look for stripped teeth in the gears inside the transmission. Remove the driven gear housing (bullet). There is a plastic driven gear on the bullet and a plastic drive gear on the transmission shaft that turn the cable.

If they look good the next thing is to make sure the cable is inserted all the way into the back of the speedometer in the dash. Sometimes the part that screws in gets crossed threaded and the square end of the cable does sit all the way in and won't spin the speedometer. I check it by removing the speedometer bullet (drive gear housing) and with the transmission cable attached, spin it to see if the speedometer needle jumps (you will need someone to help you).

If those checks are good and it still won't move than the speedometer itself is bad and needs to be replaced.