1993 Geo Prizm Speedometer

First check with GM or Toyota to see if the speedometer gauge is available. You may have to buy the complete IP.
Get a Haynes manual for your model, it will give detailed instructions on removal and installation of the IP.

If the speedometer is not working and it is cable driven, the cable (lower one) is broke.
Check the trans. There would be a cable coming out of the trans. And connects to another cable that goes into the IP cluster, usually the lower cable breaks. You can get a new lower cable from Toyota.