Generally the cruise control amplifier/brain that is mounted under the drivers side kick panel. This is assuming that the cable is hooked up from the vacuum servo and the servo is good. Signs of the amp being bad is the cruise will engage when the lever is moved but won't hold when let go.

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My Truck.. 2007 DODGE, 5.9 Cummins, 6spd stick, 4X4. My car..1977 240D, OTHER WHEELS...1955 VW Oval window bug, European Delivery (Holland) with a 1700cc, 2 barrel, Porsche drum brakes. 1939 WILLYS Pick-up. 1967 Triumph 200cc Tiger Cub. 1976 Honda 550F 4cyl Motor Cycle.