The cruise control has a couple of vacuum switches - one for the brake and one for the clutch. IF either of these is mis-adjusted, the cruise will not work. A quick check - while driving, lift up on the pedals with your feet and try to engage the cruise. If it works, re-align the switches according to the manual. If this does not work, you will need to dig deeper. Some cruise systems are equipped with separate electrically operated vacuum pump (especially turbos), and sometimes these vacuum pumps fail. Sometimes the stalk mounted switches or the connectors go bad and either need cleaning or replacement.