FLC OF AN MLG FAILED TO TURN ONTO THE LDA FINAL COURSE FROM AN ATC BASE LEG VECTOR RESULTING IN A TCASII RA AND INTERVENTION BY THE CTLR.

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WE WERE GIVEN RADAR VECTORS TO THE LDA RWY 12L AT STL. THE CTLR WAS VERY BUSY; GIVING US SEVERAL DIFFERENT STEP DOWN ALTS; CHANGING HIS MIND ON THEM A COUPLE OF TIMES. HE TURNED US ON A BASE LEG (210 DEGS) ABOUT 11 MI OUT AT A FAIRLY HIGH ALT. I WAS TRYING TO GET THE AIRPLANE CONFIGURED TO GET DOWN WHEN WE RECEIVED A TCASII 'TFC' ABOUT THIS TIME. I REALIZED WE SHOULD BE TURNING INTO THE LDA AND THAT THE 'TFC' WAS ON THE PARALLEL ILS. ABOUT THIS TIME THE CTLR SAID 'ARE YOU GUYS GOING TO START YOUR TURN?' MY CAPT RESPONDED 'WE NEVER GOT ONE' (I NEVER HEARD ONE GIVEN). AT THIS TIME WE GOT A 'CLB; CLB NOW' TCASII AND THE CTLR TOOK US OFF THE APCH AND WE CLBED. WE WERE NEVER LESS THAN 1000 FT FROM THE OTHER ACFT. THIS LDA APCH IS A VERY DEMANDING APCH DURING BUSY TIMES. I THINK THE WHOLE LDA SYS AT STL NEEDS TO BE LOOKED AT. ANOTHER THING THAT WOULD HELP IS A NDB ON THE LDA. THIS WOULD REALLY HELP IN YOUR SITUATIONAL AWARENESS. IF WE WOULD HAVE HAD THIS AID; I WOULD HAVE SEEN WE WERE GETTING CLOSE TO THE LDA COURSE AND WOULD HAVE QUESTIONED THE TURN ON.