ARPT PROBLEM ILS CREATES A PROBLEM FOR ACR FLC IN HDG TRACK DEV THAT NEARLY RESULTED IN ACFT LNDG ON TAXIWAY.

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ON ILS FINAL APCH TO 18L AT MEMPHIS; WE HAD THE RWY IN SIGHT AT ABOUT 1000 FT AGL. DRIFTING FOG; CLOUDS; AND LIGHT RAIN MADE KEEPING THE RWY IN SIGHT AT ALL TIMES DOUBTFUL; SO I TOLD THE COPLT THAT I WOULD STAY ON THE GAUGES. AT ABOUT 500 FT AGL; 2 RED LIGHTS CAME ON ON MY INST COMPARATOR; AS WELL AS THE COPLT'S COMPARATOR. OUR ATTN WAS IMMEDIATELY DRAWN TO THE RED LIGHTS AND THEN TO THE ILS RAW DATA. ABOUT THE TIME WE HAD CONCLUDED THAT WE HAD NO COURSE OR GS GUIDANCE; TWR ADVISED THAT THEY HAD ILS WARNINGS AND IF WE DID NOT HAVE THE RWY IN SIGHT TO GAR. LOOKING UP; WE WERE EXACTLY LINED UP WITH THE PARALLEL TAXIWAY JUST TO THE R OF THE RWY. WE HAD BEEN TOLD TO EXPECT ONE DEP PRIOR TO OUR ARR; AND SITTING RIGHT IN THE MIDDLE OF THE TAXIWAY WAS AN AIRPLANE; AS IF IN TKOF POS. THINKING WE WERE STILL LINED UP WITH THE RWY; WE CALLED RWY IN SIGHT AND WERE CLRED TO LAND. OUR ATTN WAS COMPLETELY CONCENTRATED ON THE ACFT AHEAD OF US SINCE HE DID NOT APPEAR TO BE ROLLING. AS I PREPARED MENTALLY FOR A GAR I TOLD THE COPLT TO ADVISE THE TWR OF THE AIRPLANE ON THE RWY. ALTHOUGH CONCENTRATING ON THE AIRPLANE ON THE GND; THE WHOLE PICTURE FELT WRONG TO ME. WHEN THE TWR RESPONDED THAT THERE WAS NO AIRPLANE ON THE RWY; I IMMEDIATELY REALIZED WHAT WAS WRONG WITH THE PICTURE; LOOKED SLIGHTLY L AND SAW THE RWY IN FULL VIEW. TWR TOLD US TO GAR. BUT WE ADVISED THE RWY IN SIGHT AND THEY CLRED US TO LAND. WE WERE ABOVE 200 FT AGL AND A SMOOTH TRANSITION AND LNDG WAS MADE. LOW VISIBILITY COMBINED WITH THE DISTR OF AN 'ACFT ON THE RWY' KEPT US FROM SEEING THE RWY WHEN WE FIRST WENT VISUAL. LOC FAILURE AND RED LIGHTS IN THE COCKPIT AT A VERY INOPPORTUNE TIME CAUSED US TO DRIFT SLIGHTLY R AS WE ANALYZED THE SITUATION. HAVING THE RWY DIMLY IN SIGHT AT 1000 FT AGL; AND EXPECTING A DEP PRIOR TO OUR ARR; CONTRIBUTED TO OUR CONFUSION. THE DECK WAS STACKED AGAINST US; BUT AN ACCIDENT WAS NOT ABOUT TO HAPPEN AS NOTHING FELT RIGHT ABOUT WHAT WE WERE SEEING AND WE WERE JUST ABOUT TO GAR FOR ANOTHER TRY.