The cruise control failure is often just an underused stalk control. They frequently spend years not being used, and they seem to corrode over slightly on the contacts.

You often find that if you simply slide the switch on top back and forwards for a minute with some vigour, and press the button in and out a good number of times, it will start working. If this fails, it may be time to change the stalk over.

On prefacelift cars, it could be a perished pipe as the cruise system is the older 'sucking' system where a pump sucks the throttle in as required to achieve the speed desired. (you can feel the throttle moving beneath your feet) There is a mechanical failsafe as the tube used for the throttle pulling is physically terminated at the brake pedal. When the brake pedal is pressed, a valve opens and the pressure in the pipe is lost preventing the cruise system from artificially accelerating the car.

If there are any leaks in this pipe system, it will never pressurise and cruise can never function.

This doesnt affect facelift cars as they have electronic throttles, and all the cruise system is built into the ECU - The cruise stalk and its wiring to the ECU is the only possible cause of failure.