What year is your car? Is the 4 speed an Auto or Manual? Was the 4 speed originally in a fiero? Was this an auto to manual or Vice Versa swap?
Code 24 could be a few issues.
Normal operation is VSS sends a PPM (Pulse Per Mile, which is 4000 pulses per mile) signal to the Speedometer. The Speedometer reduces the signal to a PPM/2 (2000PPM). The reduced pulse signal is then sent to the ECM and Cruise Control.
The ECM giving a code 24 means the Speedometer or the VSS is not working. Check your connections, starting at the VSS. Check the wires (purple with white stripe and a solid yellow). Check the black connector at the bottom of the speedometer.
The VSS can be removed and driven by a variable speed (0-4000rpm) drill to check for voltage. Use a volt meter set to AC-24V.
[This message has been edited by IFLYR22 (edited 01-24-2014).]