THE PLT LISTENED TO WX RPRTS ON THE RADIO, BUT DIDN'T RCV AN FAA OR NWS BRIEFING. A FRONTAL SYS WAS FORECASTED TO ACROSSHIS RTE OF FLT WITH PRCTNS FOR MULTIPLE CLOUD LAYERS, LOW CEILINGS, RESTRICTED VIS, ICING & RAIN SHOWERS. THE PLT TOLD A FRIEND, WHO DROVE HIM TO THE ARPT, THAT HE COULD 'FLY UNDER THE HEAVY CLOUDS' THAT THEY OBSERVED TO THE SOUTH. THE ACFT DEPD WATSONVILLE AT ABT 1450 PST, BUT DIDN'T ARRIVE AT THE DESTN (SANTA MONICA); 15 DAYS LTR, IT WAS FND ON ANACOPAISLAND (14 MI WEST OF OXNARD), WHERE IT IMPACTED A 1112' RIDGE AT ABT THE 440' LVL. RADAR DATA SHOWED A PROBABLE TRACK LEADING TWD THE ACDNT SITE, BUT IT HAD DISAPPEARED ABT 22 MI NW OF THE CRASH SITE, AFTER THE ACFT HAD FLOWN WEST OF A MORE DIRECT RTE (OVER OCEAN WATER). A HELICOPTER PLT, WHO WAS IN THE VCTNY AT THE TIME, RPRTD A 500' BROKEN TO OVERCAST SKY COND WITH LESS THAN 2 MI VIS & MOD RAIN SHOWERS. THE ACDNT WAS ESTIMATED TO HAVE OCCURRED AT ABT DUSK. NO PREIMPACT PART FAILURE/MALFUNCTION OF THE ACFT OR ENG WAS FND.