THE PILOT STATED WHILE LANDING TO THE SOUTH, THE AIRPLANE TOUCHED DOWN ON THE 1,700-FT WET GRASS STRIP APPROXIMATELY 700 FT FROM THE DEPARTURE END. AS THE BRAKES WERE APPLIED, THE AIRPLANE BEGAN TO SLIDE. THE AIRPLANE RAN OFF THE END OF THE RUNWAY, AND IMPACTED A CULVERT THAT PARALLELED A ROAD AT THE END OF THE RUNWAY. THE AIRPLANE NOSED OVER AND CAME TO REST INVERTED ON TOP OF A PICKUP TRUCK THAT HAD BEEN TRAVELING ALONG THE ROAD. A CAR THAT WAS ALSO TRAVELING ALONG THE ROAD STRUCK THE AIRPLANE, INJURING ONE OCCUPANT OF THE CAR.