DRIVER PULLED TRUCK INTO SLIGHTLY INCLINED DRIVEWAY AND SHIFTED INTO PARK. VEHICLE WAS IDLING. BOTH DRIVER SIDE AND PASSENGER SIDE WINDOWS WERE OPEN. DRIVER PUT DRIVER SIDE POWER WINDOW UP. THE BUTTON TO RAISE THE PASSENGER WINDOW FROM DRIVER SIDE DOES NOT WORK. SO DRIVER GOT OUT OF VEHICLE, LEFT THE DRIVER SIDE DOOR OPEN AND WALKED AROUND REAR TO GET TO PASSENGER SIDE. PASSENGER WINDOW WAS OPEN. DRIVER UNLOCKED PASSENGER DOOR, OPENED IT, AND PUT POWER WINDOW UP. DRIVER LEFT PASSENGER DOOR OPEN AS THERE WAS SOMETHING ON THE SEAT TO PICK UP ONCE VEHICLE WAS OFF. DRIVER WALKED BACK AROUND VEHICLE. AS THE DRIVER WAS APPROACHING THE DRIVER SIDE TO TURN THE VEHICLE OFF THE VEHICLE STARTED DRIFTING BACKWARDS. DRIVER TRIED TO STOP VEHICLE BUT MOMENTUM WAS TOO STRONG. OPEN PASSENGER DOOR STRUCK WOODEN MAILBOX POST AND VEHICLE STOPPED. VEHICLE DRIFTED APPROXIMATELY 8-10 FEET. DAMAGE IN FORM OF DENTS WAS INCURRED TO PASSENGER DOOR. DRIVER CHECKED TO SEE IF MAYBE HE HAD LEFT THE VEHICLE IN DRIVE, OR SHIFTED TO REVERSE, NOT PARK. DRIVER CONCLUDES THE VEHICLE WAS IN PARK. TO CHECK THIS, THE DRIVER REENACTED THE SEQUENCE WITH THE VEHICLE IN BOTH DRIVE AND IN REVERSE AT THE SAME POSITION ON THE DRIVEWAY. THE VEHICLE LEFT IN DRIVE DID NOT MOVE. THE VEHICLE LEFT IN REVERSE IMMEDIATELY DRIFTED BACKWARDS. THE UNKNOWN IS THAT IN THE ORIGINAL INCIDENT THE DRIVER ESTIMATES THAT THE VEHICLE WAS IN PARK FOR NEARLY ONE MINUTE, IF NOT MORE, BEFORE DRIFTING. DURING THIS SPAN