Cruise control
Another thought: It used to be that the cruise controls sensed speed from a coil looking at a magnet attached to the driveshaft.
In later years, in most applications, they used a direct connection to the vehicle speed sensor line.
It sounds as though that connection is not hooked up. The cruise control will power up, but is searching for a clue as to the speed of the vehicle. Finding no (or what appears as invalid) data, it failsafes by disengaging.
Bob