DC10 CREW RECEIVES TCASII RA AFTER LOSING SIGHT OF TFC ON APCH TO LAX.

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WE DEPARTED SMO VOR ON HDG OF 070 DEGS WITH INSTRUCTIONS TO DSND TO 2500 FT ON THIS HDG. WE WERE TOLD TO FOLLOW A DC8 6 MI AHEAD FOR A VISUAL TO RWY 24R. AS THE DC8 PASSED ABEAM US; WE STARTED A R BASE TO FOLLOW HIM. DURING THE TURN THE APCH CTLR CLRED US FOR THE VISUAL TO RWY 24L. A FEW SECONDS LATER; THE CTLR ASSIGNED US A 220 DEG HDG TO INTERCEPT THE VISUAL FINAL FOR RWY 24L. WE SAW PARALLEL RWYS WE ASSUMED WERE RWYS 24L/R; WHICH IN REALITY WERE RWYS 25L/R. ABOUT THAT TIME WE RECEIVED A 'DSND RA' FROM THE TCASII; WHICH WE COMPLIED WITH. WE REALIZED OUR ERROR AS THE CTLR ASSIGNED US A 280 DEG HDG TO JOIN FINAL FOR RWY 24L. THE CTLR THEN TOLD US OF TFC ABOVE US. THE REST OF THE APCH WAS UNEVENTFUL. SEVERAL FACTORS CONTRIBUTED TO THE PROB. FIRST; WE WERE CLRED A VISUAL APCH FOLLOWING TFC TO A RWY WITH INOP LOC AND OM. SINCE THE LOC WAS INOP FOR OUR ASSIGNED RWY; WE HAD LAX VOR TUNED. THE SECOND PROB WAS WHEN THE CTLR SWITCHED US OVER TO RWY 24L; WE WERE OUTSIDE OF THE VISIBILITY LIMITS TO SEE THE COMPLEX. WHILE WE WERE TRYING TO ACQUIRE RWY 24L VISUALLY; WE LOST VISUAL CONTACT WITH THE DC8. THE CTLR THEN ASSIGNED THE HDG 220 DEGS; AND AS WE PROCEEDED ON THE VECTOR; WE VISUALLY ACQUIRED RWYS 25L/R; WHICH WE MISTOOK FOR RWYS 24L/R. THIS INCIDENT COULD HAVE BEEN PREVENTED HAD THE CTLR NOT ASSIGNED A VISUAL APCH BEFORE CONFIRMING THAT WE HAD RWY 24L IN SIGHT. CTLRS MUST REALIZE THE DIFFICULTY IN ACQUIRING TFC/ARPT AT NIGHT IN THE LAX TERMINAL AREA. THEREFORE; LAST MIN CHANGES; COMBINED WITH NAVAID OUTAGES; SHOULD BE AVOIDED. WHEN IT BECOMES OBVIOUS TO THE CTLR THAT AN ACFT IS DEVIATING TO THE WRONG COMPLEX AT LAX; A CORRECTION SHOULD BE ISSUED IMMEDIATELY. AS FOR THE CREW; WE SHOULD HAVE TUNED THE LOC FOR RWY 24L IMMEDIATELY UPON RECEIVING CLRNC FOR THAT RWY INSTEAD OF TRYING TO ACQUIRE THE RWY BY VISUAL MEANS.