The speedometer system is simple. You have the gauge on your instrument cluster. In the back of it, there is a cable (think like a bicycle cable) plugged into the rear of it. Said cable runs down and plugs in somewhere near the front end of your driveshaft. my truck is an automatic, and the cable is plugged into the top of my tranny. It should be easy to find.
Typically, the speedometer is either unplugged at either end, OR, the cable is broken somewhere.