WHILE TRAVELING ON THE HIGHWAY WE SAW LOOKED OUT THE REARVIEW MIRROR AND SAW WHITE/GRAY SMOKE COMING UP FROM BETWEEN THE TRUCK BED AND THE CAB OF THE TRUCK. WE EXITED THE FREEWAY. WHILE EXITING THE WHITE/GRAY SMOKE TURNED TO BLACK THEN FLAMES SHOT UP. WE PULLED OVER AND GOT OUT AS THE FLAMES CONTINUED TO GROW. WE ATTEMPTED TO PUT OUT THE FIRE BUT IT GREW VERY FAST. THE BACK GLASS EXPLODED AND THE ENTIRE CAB AND FRONT OF THE VEHICLE WAS ENGULFED. 911 WAS CALLED. FIREFIGHTERS ARRIVED AND PUT OUT THE FIRE. THE VEHICLE AND CONTENTS WERE A TOTAL LOSS. THE FIRE DEPARTMENT REPORTS STATES THAT THE FIRE WAS CAUSED BY A MECHANICAL FAILURE IN THE ENGINE COMPARTMENT OR UNDER THE PASSENGER COMPARTMENT CAUSING FLAMES TO SHOW FROM THE BACK OF THE VEHICLE AND CAUSING HEAVY DAMAGE TO THE FRONT OF THE VEHICLE AND PASSENGER COMPARTMENT.