Are you saying that the wipers won't park or are the wipers parking and the wiper cover isn't closing? The switch for parking the wiper arms is built into the wiper motor, and if the arms are not parking, the problem could be in the motor. If the arms are parking and the cover isn't closing, then check the cover limit switch. If you remove the grill on the wiper cover, on the passenger side, there is a plastic vacuum limit switch that makes contact with the wiper arm. When the arm makes contact with this switch, it "tells" the cover that the arms are parked and the cover can close. This switch tends to come apart or fail with age.