ACR LANDS ON PARALLEL RWY TO WHICH THEY WERE CLRED.

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FLT FLOWN FROM HOU TO SAT AT FL280. CLRED TO FLY MARCS 1 ARR. AT CRISS INTXN CLRED TO HDG 290 DEGS AT 6000 FT. EXPECT VISUAL TO 12R; SAT. TFC CALLED ON FINAL AND WAS SEEN BY CREW. ACFT TURNED TO APPROX 190 DEGS; BASE LEG AND CLRED VISUAL TO 12R. CAPT WAS FLYING AND HAD ALREADY BEGUN TO CONFIGURE AS WE WERE CLOSE IN AND HIGH. BEGAN DSCNT TO FINAL AND TURN TO RWY. RWY WAS DOWN IN CITY LIGHTS AND AS WE TURNED; I BEGAN TO LOOK FOR RWY. NOTE: CAPT AND I BOTH HAD ILS TUNED; BUT PICTURE WAS NORMAL FOR SITUATION; I.E.; HIGH (ABOVE GS) AND L OF LOC. AS WE ANGLED TO FINAL; CAPT ASKED FOR ME TO CHK ILS FREQ. IT WAS OK. I THEN CHKED TO SEE WHAT APPEARED TO BE SMALL RWY TO L. THEN MADE MANDATORY ALT CALLOUTS (1000; 500; 4; 3; 2; 1; ETC). SAW WE WERE ON PROPER GS (VASI). DID NOT REALIZE WE HAD LANDED ON 12L UNTIL TURNING OFF. LNDG WAS NORMAL AS WAS SLOW DOWN TO TAXI SPD. CONTRIBUTING FACTORS: HIGH APCH; SHORT FINAL; RWY LIGHTS TO 12L WERE UP HIGH (NEVER NOTICED 12R LIGHTS); DON'T KNOW WHAT APPEARED TO ME AS 12L LIGHTS ON FINAL. DISCOVERED BY CAPT. CORRECTIVE ACTION: WHENEVER ONE FACTOR SEEMS OUT OF PLACE; I.E.; CHK ILS FREQ; INVESTIGATE AND GAR IF NECESSARY. ALSO; CHK RWY DESCRIPTION THOROUGHLY AS 12R AT SAT DID NOT HAVE VASI. ALSO; APCH AND TWR CTLRS SHOULD DOUBLECHK ACFT ON FINAL REGARDLESS OF WX AND/OR HDOFS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES FLT WAS KEPT VERY HIGH ON DOWNWIND. L TURNS; FO UNABLE TO SEE (FIND) ARPT. ON BASE STILL VERY DIFFICULT TO SEE ARPT. LOOKING FOR TFC THEN HEARD TFC CLRED TO TAXI SO BACK TO CHKING ON APCH. THE PICTURE LOOKED CORRECT DUE TO HIGH APCH; CLOSE IN TURN. AFTER LNDG TWR CLRED TO CROSS RWY TO RAMP. THEY WERE NOT EVEN AWARE OF WRONG RWY LNDG. THIS WAS NOT NORMAL LNDG DIRECTION. CREW HAD NOT LANDED THIS RWY BEFORE. MOSTLY THE BRIGHT LIGHTS ON L RWY AND DIM OR NO LIGHTS ON R RWY CAUSED CREW TO LOCK IN ON THE WRONG RWY. FEELS SEVERE CLR MAY HAVE LULLED EVERYONE INTO THINKING 'NO PROBLEM.'