ON 10/6/2015, I HAD MY NEPHEW IN MY TRUCK WHILE I WAS INSTALLING HIS CAR SEAT. THE CAR WAS NOT RUNNING, THE KEYS WERE IN MY POCKET. AS I WAS INSTALLING THE CARSEAT , HE WAS ABLE TO PULL MY SHIFTER OUT OF PARK CAUSING THE VEHICLE TO ROLL BACKWARDS DOWN THE STREET. AS I JUMPED INTO PASSENGER SIDE TO REACH THE BRAKE, THE FRONT PASSENGER DOOR HIT A BRICK MAILBOX AND CAUSED DAMAGE TO THE DOOR AND FRONT PASSENGER SIDE FENDER. THE BRAKE SHIFT INTERLOCK THAT SHOULD KEEP THIS FROM HAPPENING IS NOT FUNCTIONING IN MY VEHICLE, AFTER THE VEHICLE WAS STOPPED FROM ROLLING, I GOT BACK IN TO SEE WHAT HAPPENED AND I CAN CLEARLY MOVE MY SHIFTEROUT OF PARK AND INTO REVERSE, NEUTRAL AND DRIVE WITH THE TRUCK TURNED OFF AND WITHOUT DEPRESSING THE BRAKE.