B757 CREW MADE A GAR WHEN THEY SPOTTED AN ACFT HOLDING IN POS ON THE RWY THEY WERE CLRED TO LAND ON.

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ARRIVING FROM MAUI AT XA00 ON SADIE 6 ARR; WE WERE TO LAND TO THE W DUE TO REDUCED CEILINGS AND VISIBILITY AT LAX WITH A 6 KT TAILWIND (NORMALLY LAND E AT XA00). ON DOWNWIND N OF ARPT; APCH CTLR RPTED THE S COMPLEX WAS DECREASING (LESS THAN CAT I). WE WERE CLRED ILS RWY 24L. MY TFC WAS (I BELIEVE) AN ACR HVY ON FINAL. AS I TURNED BASE; APCH WAS NORMAL. AT 500 FT AGL CAPT CALLED 500 FT; FINAL FLAPS 30 DEGS. I RPTED AND HE CONFIRMED. I LOOKED DOWN THE FULL LENGTH OF RWY 24L AND RWY WAS CLR (GOOD VISIBILITY). SUDDENLY I NOTICED A SMALL ACFT SITTING ON THE NUMBERS 24L. (COULD HAVE BEEN A COMMUTER OR CPR; I DON'T KNOW FROM MY VIEW -- ALL DARK EXCEPT FOR WHAT APPEARED AS ALMOST 'WIG-WAG' LIGHTS AT WINGTIPS.) I MADE AN IMMEDIATE GAR. THE ACFT WAS VERY DIFFICULT TO SEE WITH THE BRIGHT LEAD-IN LIGHTS; END LIGHTS; ETC. WE HAD BEEN CLRED TO LAND BY THE TWR 'NORMALLY.' NO MENTION OF ANY ACFT IN POS PRIOR TO OUR ARR. I AM SUBMITTING THIS NASA RPT NOT TO POINT BLAME (I HAVE BEEN THERE TOO!); BUT IN LIGHT OF THE ACCIDENT ON RWY 24L SEVERAL YRS AGO LNDG ON TOP OF AN ACFT IN POS (INTXN ALTHOUGH; I BELIEVE). WE MUST INVESTIGATE HOW THE 'SYS' FAILED TODAY. HAD THE VISIBILITY BEEN AS LOW AS RWY 25L (CAT II OR CAT III); WE MIGHT HAVE HAD A SERIOUS ACCIDENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: PLT STATED THE ACFT HOLDING IN POS WAS AN ACR B757. THE TWR WAS STAFFED FOR SINGLE CTLR OPS FOR BOTH THE N AND S SIDE. THE S SIDE WX HAD JUST GONE TO CAT II OR CAT III. THE TWR CTLR WORKLOAD WAS HIGH DUE TO THIS. AFTER THE GAR THE TWR CTLR WOULD NOT TALK ABOUT THE INCIDENT OVER THE RADIO. AFTER LNDG THE GND CTLR TOLD THE CREW THAT THE B757 WAS CLRED INTO POS; AND THEN THEY JUST FORGOT ABOUT HIM. PLT GROUP SAFETY WAS NOTIFIED. THEY DISCOVERED THAT NO FAA INQUIRY WAS IN PROGRESS. THERE WAS A SIMILAR INCIDENT AT LGA. THE PLT THINKS THE SYS SHOULD BE FIXED.