where are you looking on the gearbox ,if it has one on the gearbox it is round the back  part of the gearbox behind the engine you will see a wiring  loom going to the sensor , but if its not there its possable the abs computer is  providng the speed signal for the vehicle  and if that is the case you will need to diagnose the abs computer  and check the parameters so you can determin the cause of the fault, ie is it a computer fault , wheel speed sensor fault on the front , ring gear on driveshaft  broken , wiring fault, instrument panel fault , all these things need to be checked to find the fault  , happy dianostics , . Or you just  might not have seen the sensor on the gearbox , then all you have to do is fit the new sensor , but check the connector make sure its in good condition , hope that helps .
                 mark
                
 May 2008