INSPECTION PROCEDURE 4: Auto-cruise Control cannot be Set.: Comment
WARNING: This page is about a different car, the 2005 Mitsubishi Montero. However, it is still accessible from the selected car via links, so may be relevant.
The fail-safe function is probably cancelling auto-cruise control. In this case, scan tool MB991958 can be used to check the trouble symptoms in each system by checking the diagnostic trouble codes. The scan tool can also be used to check if the circuits of each input switch are normal or not by checking the input switch codes.