THE NON-INSTRUMENT RATED PLT WAS ON A FLT FROM FARMINGTON, NM TO EL CAJON, CA, BUT MADE A STOP AT BORREGO SPRINGS, CA WHERE A PASSENGER DEPLANED. BEFORE DEPARTING FARMINGTON, HE OBTAINEND A WX BRIEFING FOR THE TRIP, BUT ONLY FILED A FLT PLAN FOR THE 1ST LEG WHEN TOLD OF BAD WX ALONG THE 2ND LEG. AT BORREGO SPRINGS, NO AVIATION FUEL WAS AVAILABLE. THE PLT WAS CONCERNED ABOUT HAVING ENOUGH FUEL FOR THE REMAINDER OF THE FLT SO HE OBTAINED 2-1/2 GAL OF SUPREME UNLEADED AUTO FUEL. ON THE NEXT LEG OF THE FLT, THE ACFT CRASHED IN MOUNTAINOUS TERRAIN. A WITNESS SAID HE HEARD THE ENG SPUTTER THEN SAW THE ACFT COME OUT OF THE CLOUDS IN A 65 DEG ANGLE. HE ESTIMATED THE BOTTOM OF THE CLOUDS WERE ABOUT 400 TO 500 FT AGL. ALSO, HE DESCRIBED THE WX AS RAINY, HEAVY AT TIMES, WITH LOW CEILINGS & SOME BREAKS IN THE OVERCAST. A TEARDOWN OF THE ENG REVEALED NO EVIDENCE OF A PREIMPACT, MECHANICAL FAILURE. THE ACFT WAS TOO BADLY DAMAGED TO DETERMINETHE AMOUNT OF FUEL THAT WAS IN THE TANKS PRIOR TO IMPACT. THE ELEVATION OF THE CRASH SITE WAS ABOUT 3900 FT.