ACR MLG CONTINUED ON RADAR VECTOR PASSING THE RWY CENTERLINE FOR INTENDED RWY AND INTRUDING ON TFC TO PARALLEL RWY.

Narrative

THE CAPT AND I HAD REMARKED ABOUT THE CTLRS WORKING MULTIPLE FREQS (BOTH AT CENTER AND APCH); EVIDENTLY SO MORE PEOPLE COULD BE AT HOME. WE WERE BEING VECTORED TO AN ILS APCH TO 21L AT DTW. STILL IMC AT 4000 FT MSL; WE WERE SET UP FOR THE PUBLISHED APCH; THE CAPT WITH THE ILS AND ME WITH CARLTON VOR; FOR DME INFO. HAVING FLOWN THAT APCH COUNTLESS TIMES. WE WERE EXPECTING ANOTHER VECTOR TO PERHAPS 240 DEGS AND THEN CLRNC TO INTERCEPT THE LOC AND THEN ILS APCH CLRNC. THE CTLR SOUNDED A BIT HURRIED AND SOME CALLS WERE BLOCKED AND THUS REPEATED. THE COURSE BECAME ACTIVE AND THEN WE FLEW RIGHT THROUGH THE LOC; STILL ON A 300 DEG HDG; 90 DEGS FROM THE INBOUND COURSE. I CERTAINLY WANTED TO SAY SOMETHING; BUT THERE WAS A CTLR-PLT EXCHANGE GOING ON. AT FULL-SCALE DEFLECTION; THE CTLR THEN TOLD OUR FLT NUMBER THAT WE WERE CLRED 'FOR THE APCH'. AT THIS POINT OUR TCASII WENT OFF AS WE WERE APPROX ON EXTENDED CENTERLINE FOR 21R. THE CTLR TOLD US THAT WE MUST HAVE MISSED EARLIER INSTRUCTIONS TO INTERCEPT AND TO DSND TO 3000 FT. WE DID; ACQUIRED THE ARPT AND ACCEPTED A VISUAL APCH; WHILE OFFSET 1-2 MI FROM 21R FINAL. ALTHOUGH OTHER ACFT WERE SEEN; NON WERE DANGEROUSLY CLOSE.