THE PLT HAD A CABIN NR MAMMOTH/JUNE LAKES ARPT & OFTEN FLEW THERE ON WEEKENDS. WHILE ON A NGT FLT, HE COMMUNICATED WITH AN OUTBOUND CARGO PLT, WHO WARNED OF HVY SNW FALL & POOR VIS AT THE ARPT. WHILE CONVERSING, THE PLT OF N5831J REPLIED, 'OH DEAR, MY CAR IS UP THERE.' THE CARGO PLT AGAIN WARNED THAT HE DOUBTED IF N5831J COULD LND AT THE ARPT. A WORKER, WHO LEFT THE ARPT AT 2030 PST, NOTED THE RWY LGTS CAME ON AS HE WAS DRIVING HOME. HE BELIEVED THE LGTS WERE ACTIVATED BY A PLT (BY KEYING THE MIKE 5 TIMES ON UNICOM FREQ). THREE WITNESSES SAW THE ACFT (OR ACFT LGTS) AT ABOUT THE TIME IT CRASHED. IMPACT OCCURRED IN A LEFT WING LOW, NOSE DWN ATTITUDE. ONE WITNESS THOUGHT THE ACFT WAS TURNING OR ARCING WHEN IT CRASHED. ANOTHER WITNESS ESTIMATED THERE WAS A 100' CEILING WITH SNW FALLING ON A DARK NGT. IMPACT WAS ABOUT 1 MI FM A HIWAY BTN MAMMOTH LAKES & BISHOP, CA (ABT 10 MI SE OF MAMMOTH/JUNE LAKES ARPT & 17 MI NW OF BISHOP ARPT). NO PREIMPACTPART FAILURE/MALFUNCTION OF THE ACFT OR ENG WAS FND. THERE WAS NO PUBLISHED INST APCH PROC AT MAMMOTH/JUNE LAKES ARPT.