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Read it discussed on another forum. Concept is straight forward, need drive train RPM sensor, not engine RPM as transmission changes ratios, and final control element to manipulate engine throttle. A user interface is needed to turn on and off and set desired cruise speed.
Most discussion was on safety and liability of such a DIY concept, there are risks to such an application. Some posters thought that buying aftermarket devices would be the more safe way to go.