ACR MD83 DEVIATES FROM IAP LDA RWY 12L PROC AT STL ARPT AS FO; PF; FOCUSES ON WRONG RWY; CAUSING PARALLEL ILS TFC TO GAR.

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STL; MARGINAL VMC-IMC; EXECUTING SIMULTANEOUS PARALLEL (SIDE-BY-SIDE) APCHS LDA DME RWY 12L; AND ILS RWY 12R. CREW PLANNED FOR ILS RWY 12R AT STL; THEN AT 15 MI FINAL APCH SWITCHED US TO LDA RWY 12L; AT 8 MI FINAL APCH TOLD US TO BREAK OFF LDA LOC AND TURN L; TO SET-UP FOR ANOTHER DOWNWIND LDA RWY 12L. THEN RADAR VECTORS TO LDA RWY 12L 10 MI FINAL; CLRED APCH RWY 12L LDA (IMC); AND CTLR TOLD US TO MAINTAIN 170 KIAS TO POTSY (FINAL APCH FIX AT 6.2 DME). THIS PUT US FAST AT FINAL APCH FIX; IN IMC; WHEN WE SHOULD HAVE BEEN PROPERLY SLOWED TO 130 KIAS WITH FULL FLAPS (40 DEGS) AND LNDG GEAR DOWN: THUS; AT THE FINAL APCH FIX CREW WAS OVERLOADED WITH EFFORT TO GET GEAR DOWN AND FLAPS 40 DEGS AND DSND AND SLOW. APCH CTLR NEVER TOLD US TO SWITCH TO TWR; UNTIL INSIDE MISSED APCH POINT; WHEN I WAS VISUALLY ANGLING TO RWY IN SIGHT. UNFORTUNATELY; WITH STAGGERED PARALLEL RWYS AT STL; I HAD FLOWN TOWARD RWY 12R. I NEVER SAW THE OTHER ACFT LINED UP ON RWY 12R! LESSONS: IN IMC -- THE ACFT ON THE NON-PRECISION APCH MUST BE PERMITTED TO EXECUTE NORMAL IMC PROCS: FULLY SLOWED AND CONFIGURED WITH CHKLIST COMPLETE AT FINAL APCH FIX (SO THAT PROPER DSCNT TO MDA CAN BE CONDUCTED). THUS; NO ATC SPD 170 KIAS (FAST) RESTRS AT FINAL APCH FIX. IN IMC -- CTLRS MUST BE FULLY PROFICIENT IN MONITORING THE ACFT ON PARALLEL SIDE-BY-SIDE (IMC) APCHS; AND ENSURE THOSE ACFT ARE SENT TO TWR FREQ PRIOR TO MISSED APCH; PREFERABLY PRIOR TO FINAL APCH FIX. I FLEW THE PUBLISHED LDA RWY 12L APCH FROM THE R SEAT. I NEVER SAW ANY ACFT ON MY R; BUT FLEW ON INSTS TO MISSED APCH; BECAUSE RWY OBSCURED BY FOG OR SCUD. AT MISSED APCH; I TURNED R WHEN I SAW THE RWY; BUT I HEADED FOR RWY 12R (CLOSER THRESHOLD WITH STAGGERED RWYS). THE CAPT NOTICED THAT I WAS FLYING TOWARD THE WRONG RWY; AND TOOK CTL; TURNED RAPIDLY L AWAY FROM RWY 12R; AND LANDED ON THE FAR STAGGERED RWY 12L. FACTORS: IMC WX; POOR ATC; CREW'S PERMITTING ATC TO FLY ACFT AND SIMULTANEOUS PARALLEL APCHS.