Most wiper motors have a set of points inside that allow the motor to "park", ie shut off with the blades in their parked position. If these points can not be "tripped" the motor wil continue to run. Check to see that the wiper arms are not blocked from their full travel. Sometimes leaves, twigs and other crud can get trapped below the windshield line and impeede the wiper arms. Clean out this area if necessary to see if this will fix the problem. Sometimes a sharp tap on the motor with a rubber mallet will jar stuck points, but if this does stop the motor running, the fix is only temporay and the motor will need to be rebuilt or replaced.
2001 Bonneville SSEi (retired at 365,000 Kms.)
2014 Jeep Grand Cherokee Overland (yes, I know its not a GM)