AN ACR MLG LOCKED ON TO THE WRONG ACFT WHILE MAKING A VISUAL APCH IN MARGINAL CONDITIONS.

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WE WERE ACR FLT X FROM LITTLE ROCK TO DFW ON APCH TO RWY 17L. WHILE ESTABLISHED ON A BASE LEG FROM THE NE OF THE FIELD; APCH CTL CALLED OUT TFC TO US ON SEVERAL ACFT. APCH CTL CLRED US TO FOLLOW ONE OF THE ACFT FOR A VISUAL APCH TO 17L. WE DID NOT HAVE THE FIELD IN SIGHT DUE TO FOGGY CONDITIONS. THE CAPT (PF) HAD TO 'PADLOCK' ON THE ACFT TO KEEP IT IN SIGHT. UNFORTUNATELY; HE HAD AN ACFT ON APCH TO RWY 18R (W SIDE) THAT HE 'PADLOCKED' ON AND WE ENDED UP STRAYING OVER TO THE W RWY 18R/L APCH FINAL. AT ABOUT 4-5 MI OUT OUR ERROR WAS DISCOVERED AND WE CORRECTED OVER TO THE CORRECT RWY. WHILE THIS CERTAINLY WAS OUR ERROR; I FEEL APCH CTL (AS WELL AS MOST APCH CTLS) PUSH VISUAL APCHS ON US INSTEAD OF ASKING. IN A RADIO INTENSE ENVIRONMENT; ONE CAN EASILY BE CLRED AND EXPECTED TO FLY A VISUAL APCH WHEN IT IS NOT DESIRED. THERE ARE FEW GUARANTEES THE TFC YOU ARE TOLD TO FOLLOW IS THE TFC YOU SEE. IT SHOULD BE NOTED THAT NOW THAT DFW HAS 3 TWRS; RWY IDENT CAN BE CONFUSING IN MARGINAL WX CONDITIONS.