My Dakota was similar. I have two switches that must be activated to engage the cruise control. First is the "ON" switch. When this is depressed there should be an indicator somewhere on the dash or steering wheel. In my Dakota it was on the dash. For some reason this is the switch that failed as it would not activate intermittently.
The other switch is the speed set switch. Then of course there is the brake switch.
In my E-450 based chassis RV it is the same... two hand controlled switches, an indicator on the dash and a third switch on the brake pedal. I suspect as others have indicated the control goes through the engine computer and out to the device that is connected to the throttle on the engine.
If you are inclined to test things out then I suggest you get it up to at least 40 miles per hour and press the on/off switch and look for the indicator either on the dash or steering wheel or even a little light on the switch itself.
If you get no indicator that the system is even turned on (activated) then it is most likely the steering wheel mounted activation switch. n most vehicles they are fairly easy to replace.
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Does your steering wheel look like this one?