THE PLT REPORTED THAT THE ACFT HAD AN ALTERNATOR FAILURE IN THE VICINITY OF CAJON PASS. HE REQUESTED & WAS PROVIDED RADAR VECTORS TOWARD THE RIALTO MUNI ARPT. HOWEVER, THE PLT SAW CABLE ARPT, APRX 14 MI WEST OF RIALTO, & THOUGHT IT WAS RIALTO MUNI. REPORTEDLY, THE PLT REDUCED HIS ELECTRICAL LOAD TO CONSERVE BATTERY POWER & HAD THE FRONT SEAT PASSENGER HOLD A FLASHLIGHT ON HIS INSTRUMENTS. DURING HIS APCH TO THE CABLE ARPT (WHICH HE THOUGHT WAS RIALTO MUNI), HE HEARD A CESSNA BROADCASTING THAT HE WAS ON FINAL AT RIALTO. WHEN THE PLT (OF N47980) COULD NOT SEE THE OTHER ACFT, HE DECIDED TO TURN ON THE ANTI-COLLISION LIGHT. HE REPORTED THAT THE FLASHLIGHT BEAM WAS REMOVED FROM HIS AIRSPEED INDICATOR AT A CRITICAL TIME ON FINAL APCH, CAUSING CONFUSION IN THE COCKPIT. SUBSEQUENTLY, THE ACFT WAS LANDED WITH EXCESSIVE AIRSPEED, OVERRAN THE RWY & EXPERIENCED AN ON-GROUND COLLISION WITH TERRAIN.