DURING PREFLT & INFLT WX BRIEFINGS, THE PLT WAS ADVISED THAT MOUNTAINS ALONG HIS ROUTE WERE OBSCURED BY CLOUDS & PRECIP.ALSO, HE WAS CAUTIONED ABOUT MODERATE ICING IN CLOUDS & PRECIP AT OR ABOVE THE FREEZING LVL. WHILE EN ROUTE, THE PLT OBTAINED CLEARANCE TO DETOUR AROUND CLOUDS. HE THEN REQUESTED A DESCENT TO GET OUT OF CLOUDS BECAUSE THE ACFT WAS PICKING UP RAIN & LIGHT ICE. HE WAS CLEARED TO DESCEND TO THE MIN EN ROUTE ALT (MEA) OF 9000 FT. A SHORT TIME LATER, RADAR & RADIO CONTACT WERE LOST. LATER, THE ACFT WAS FND WHERE IT IMPACTED THE SLOPE OF A HILL ON OPEN DESERT TERRAIN WHILE IN A STEEP, NOSE DOWN, RIGHT WING LOW, ATTITUDE. IMPACT OCCURRED AT HIGH SPEED ON A NORTHERLY HEADING. THERE WERE NO KNOWN WITNESSES. NO PREIMPACT PART FAILURE OR MALFUNCTION WAS FOUND. ACCORDING TO MAINTENANCE RECORDS, THE LAST PITOT-STATIC SYS, SENSITIVE ALTIMETER & TRANSPONDER CHECKS WERE PERFORMED ON 6/3/83, & THE LAST VOR CHECK WAS ON 1/6/78.