A DC9 CREW; ON A VISUAL APCH TO BNA; LINED UP ON THE WRONG RWY.

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CAPT OF ACFT ACQUIRED ARPT RWY ENVIRONMENT WHILE ON A DOWNWIND APCH. OVCST SKIES CAUSED A 'SHADOWING' EFFECT ON THE ARPT. THE FIRST RWY ACQUIRED IN SIGHT BY CAPT WAS RWY 2C; WHICH HE MISTOOK FOR RWY 2L. AS WE TURNED FINAL I SAW THE RWY FOR THE FIRST TIME. (NOTE: I WAS SITTING ON R SIDE OF ACFT THEREFORE I COULD NOT SEE THE ARPT VISUALLY UNTIL ROLLING OUT ON FINAL. INITIALLY THE HSI AND GS INDICATIONS SHOWED THAT WE WERE ON COURSE FOR THE CORRECT RWY DUE TO THE CLOSE PROX OF RWYS 2L AND 2C TO EACH OTHER. AS WE APCHED THE RWY; THE INST GS DID NOT MATCH THE VISUAL GS. WE IMMEDIATELY QUERIED THE CTLR AND HE INFORMED US THAT WE WERE ALIGNED WITH RWY 2C. HE IMMEDIATELY INDICATED 'NO PROB' AND CLRED US TO LAND ON RWY 2C. SINCE WE WERE SAFELY ESTABLISHED ON THE APCH WE CONTINUED AND A SAFE AND NORMAL TOUCHDOWN WAS ACCOMPLISHED. I BELIEVE THE OVCST 'SHADOWING EFFECT' WAS A CONTRIBUTING FACTOR IN THE MISIDENT OF THE RWY. ADDITIONALLY; I DO NOT BELIEVE VISIBILITY WAS AS GOOD AS THE TWR RPTED. INST INDICATIONS DO NOT DEFLECT MUCH UNTIL CLOSE IN TO THE RWY ESPECIALLY CDI INDICATIONS. THE RWY CONFIGN REQUIRES PLTS TO BE EXTRA VIGILANCE WHEN MAKING VISUAL APCHS IN ALMOST ANY CONDITION.