I would like to know where is the speed control sensor located on my 2004 trailblazer

Speed control sensor?
Are you sure you're not looking for Vehicle Speed Sensor (VSS)? The VSS is mounted into the transmission tailshaft housing (at the rear of the transmission).
If you're looking for Cruise Control stuff, cruise operation is controlled by the Powertrain Control Module (PCM) along with the VSS and a throttle actuator motor to control the throttle position instead of a mechanical cable.