ON NOVEMBER 05, 2016 AT APPROXIMATELY 1230 HOURS I WALKED OUT TO MY RAM 1500 PICK UP TRUCK, WHICH WAS PARKED IN MY DRIVEWAY AND STARTED IT UP. I PULLED THE TRUCK FORWARD IN MY DRIVEWAY APPROXIMATELY A FULL TRUCK LENGTH, PUT THE TRUCK IN PARK AND EXITED THE TRUCK, LEAVING THE DRIVER SIDE DOOR OPEN. THE TRUCK WAS LEFT IDLING. I BEGAN TO WALK TO THE REAR OF MY TRUCK, WHERE A FRIEND WAS STANDING NEAR MY FLATBED TRAILER. THE PLAN WAS TO HOOK THE TRAILER UP TO THE TRUCK. AS I WAS WALKING TO THE REAR OF MY TRUCK, I OBSERVED THE TRUCK ROLLING BACKWARDS. I IMMEDIATELY TURNED AROUND AND BEGAN TO RUN BACK TO THE TRUCK. AS THE TRUCK WAS ROLLING, THE DRIVER SIDE DOOR CAUGHT THE TIRE OF THE VEHICLE PARKED NEXT TO IT. AS I JUMP BACK IN THE TRUCK I CHECK THE SHIFT INDICATOR AND OBSERVED IT TO BE IN PARK WHILE ROLLING. THE DRIVER SIDE DOOR WAS THEN PUSHED BACKWARDS AND INTO THE DRIVER SIDE FRONT FENDER. I THEN STEPPED ON THE BRAKE AND THE VEHICLE CAME TO A STOP.