The speed sensor is located on the back of your transmission held by 2 bolts with a wire harness leading to it. If you still can't find it, start from your positive wire cable at your battery and follow the thick wire harness until it leads below your throttle intake. You should see a small wire branch out onto the Vehicle Speed Sensor.
When you disconnect the wire harness, you should see 3 contact points. You would need a multimeter to measure the voltage and ohms across a combination of the points to determine if it's bad. Or you could just buy a new Vehicle Speed Sensor, replace the old one and see if that resolves your problem. If it doesn't, just return the sensor for a refund.