AN ACR MLG LINED UP ON THE WRONG RWY AT DFW.

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ON A L BASE TO FINAL AT 3000 FT MSL; APCH WAS VECTORING US HEAD ON TO AN AIRPLANE ARRIVING FROM THE NW. SIMULTANEOUSLY; AS OUR TCASII WARNED US OF THE CONFLICT; THE CTLR POINTED OUT THE TFC. AS WE ACQUIRED THE TFC VISUALLY XING IN FRONT OF US AND PASSING OFF OUR L; WE GOT AN RA TO CLB. AT THE SAME TIME; THE CTLR TOLD US TO FOLLOW THE ACFT OFF TO OUR L; NOW GOING TO 7 O'CLOCK TO THE E SIDE TO LAND ON RWY 17L. THE TFC AHEAD (NOW BEHIND US) WAS TO BE LNDG ON RWY 17R. BY THE TIME WE WERE CLRED VISUAL TO RWY 17L WE WERE MIDFIELD (1/2 WAY BTWN RWY 17R AND RWY 18L) AND TURNED L TO PARALLEL A COURSE INBOUND. AS WE ROLLED OUT; WE WERE LINED UP FOR THE W SIDE FOR RWY 18L. WE CONTINUED INBOUND FOR APPROX 3 MI LINED UP WITH RWY 18L AND THE CTLR NEVER INFORMED US OF THE WRONG APCH BUT MERELY HANDED US OFF TO TWR. JUST PRIOR TO OUR CONTACTING TWR; I REALIZED THE ILS LOC WAS OFF SCALE L AND REALIZED WE WERE NOW 4 MI OUT HEADED FOR RWY 18L. AS I TURNED THE ACFT TO INTERCEPT RWY 17L; TWR INFORMED US OF OUR POS. LNDG ON RWY 17L WAS UNEVENTFUL. BAD VECTOR FROM APCH! WE WERE SET UP; AT DALLAS; AGAIN! SUPPLEMENTAL INFORMATION FROM ACN 256736: WE RESPONDED TO THE CLB RA. WE CONTINUED INBOUND UNAWARE WE WERE ON THE W SIDE OF DFW FOR APPROX 3 MI.