Is this piece(sprocket) a DIY or does one have to bring it to a shop to have this piece changed. My speedometer stopped working once the car was returned from the tranny shop. Before it went to the shop, the needle would jump around at low speeds. When I received the car back, it worked for a few days then stopped working altogether.

The cable is located at the left rear (tailshaft area) of the tranny. Held in place by a pinch fitting with a 10mm bolt & nut..The speedo driven gear and carrier are held into the tail shaft housing by a retainer. This piece, (speedo gear), as I understand it rarely fails. Most likely the cable has parted.