Does the switch lightup when the Cruise Control switch is pushed?
If not I would start by checking fuses 16 and 18. After that my next checks would include the brakelight switch (and clutch switch if a manual). Then the cruise switch and switch on steering wheel (also could be the 'rolled coil' or clockspring). Also check the vacuum line to the actuator under the hood as well as the cable from the actuator to the throttle lever. And ANY connectors, cables or components in between.
Next would be the main components, the actuator assy itself, or the cruise control module.
98S, 2014 Impreza 5 dr Ltd (9 Subaru's since 1978, 7 were 5 sp) ILuvSubaru's