HDG TRACK POS DEV BY B737-300 ON VECTORS FOR IAP ILS RWY 3L AT DTW. FLC MISSED RWY CHANGE ON AMENDED CLRNC ROUTING CHANGE.

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NORMAL FLT; ON RADAR VECTORS TO RWY 3L AT DTW. WE WERE GIVEN A 360 DEG HDG TO JOIN THE LOC; CLRED THE APCH. NEXT WE WERE GIVEN THE LCL TWR FREQ THAT NORMALLY WOULD BE USED FOR RWY 3R. CAPT AND I BOTH VERBALIZED THIS TO EACH OTHER. WHEN CAPT CHKED IN WITH THE TWR WE WERE CLRED TO LAND RWY 3R. CAPT REPLIED WE'D BEEN GIVEN VECTORS FOR RWY 3L. THE TWR IMMEDIATELY GAVE US A VECTOR FOR RWY 3R; RECLRED FOR THE APCH FOR RWY 3R AND CLRED TO LAND. THE REST OF THE APCH AND LNDG; ETC; WERE UNEVENTFUL. NO CONFLICTS OR TCASII EVENTS. AT SOME POINT DURING THE VECTORS TO RWY 3L THE APCH CTLR SWITCHED US TO RWY 3R WHICH NEITHER THE CAPT NOR MYSELF HEARD. WHEN THE CTLRS SWITCH RWYS FOR CREWS THEY NEED TO EMPHASIZE THIS CHANGE TO THE CREW AND GET AN ACKNOWLEDGEMENT FROM THE CREW. 2 WKS PREVIOUS TO DTW THE CAPT AND I HAD FLOWN THE SAME TRIP EWR TO DTW AND USED RWY 3L AT THAT TIME. SUPPLEMENTAL INFO FROM ACN 359367: AS I WAS TRYING TO IDENT THE ILS FREQ WE WERE ISSUED AN APCH CLRNC; 'FLT XYZ; TURN R HDG 360 DEGS; MAINTAIN 3000 FT; CLRED FOR THE ILS RWY 3R; CONTACT DTW TWR ON 118.4.' MY READBACK TO THE CTLR AS I AM BEST ABLE TO RECALL WAS; 'FLT XYZ HDG 360 DEGS; CLRED FOR THE APCH; TWR 118.4.' HOWEVER; WE BOTH WERE SURPRISED BY THE ASSIGNED TWR FREQ WE WERE GIVEN SINCE WE KNEW IT WAS THE RWY 3R TWR FREQ AND NOT WHAT WE WERE EXPECTING FOR RWY 3L. WHEN I CHKED IN WITH DTW TWR I EMPHASIZED THAT WE WERE ON A DOGLEG TO FINAL FOR THE ILS RWY 3L. THE RESPONSE FROM TWR WAS; 'FLT XYZ THIS IS MONITOR; TURN R TO 060 DEGS AND INTERCEPT THE FINAL FOR RWY 3R.' WHILE THE FO WAS TURNING THE JET I REMARKED TO THE CTLR THAT WE WERE ASSIGNED RWY 3L AND HE RESPONDED THAT HE RESERVED THE OPTION TO SWITCH RWYS. WHAT WAS MISSING HERE WAS A 'HEADS-UP' BY THE CTLR THAT OUR RWY WAS CHANGED TO RWY 3R. THE FIRST TIME THAT THE RWY CHANGE WAS COMMUNICATED TO US WAS IN THE APCH CLRNC AND WE (FO AND MYSELF) BOTH MISSED IT. BOTH THE FO AND MYSELF WERE SURPRISED BY THE ASSIGNED TWR FREQ CAUSING ME TO EMPHASIZE THAT WE WERE CLRED FOR RWY 3L WHEN I CHKED IN. THIS TURNED OUT TO BE GOOD. I THINK THE ACTION THAT WOULD HAVE HAD THE GREATEST IMPACT ON PREVENTING THIS EVENT WOULD HAVE BEEN AN EARLIER XMISSION FROM THE FINAL CTLR THAT WE WOULD NOW BE CONDUCTING AN APCH TO RWY 3R; IE; 'FLT XYZ YOUR RWY IS NOW RWY 3R.' HAVING RECEIVED THE CLRNC WHILE IDENTING THE ILS I PROBABLY SHOULD HAVE ASKED FOR IT AGAIN. HOWEVER; I THOUGHT I HAD IT ALL AND THE FO DIDN'T HEAR THE RWY 3R EITHER. I'LL TRY TO LEARN FROM THIS.