With the ignition and wiper switches turned on, (not delay mode), check for voltage on the metal case of the wiper motor. If there is voltage on it, the ground strap has a burned spot. Running a new ground wire between the motor and the car body sheet metal will solve that permanently.
If there's no voltage on the motor, check for voltage on the wires in its electrical connector. If nothing is there, check the fuses and the wiper switch.