ACR FREIGHT HAULER IN A NIGHT OP MAKES WRONG ARPT APCH LNDG.

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WE WERE ON AN IFR FLT TO MARION; IL. WE WERE CLRED FROM 5000 FT TO 2300 FT AND A VISUAL APCH ONCE WE HAD THE AIRFIELD IN SIGHT. IN THE START OF THE DSCNT; THE CAPT SAID I HAVE A BEACON IN SIGHT; AT WHICH TIME I ACKNOWLEDGE AND SAID I HAVE THE AIRFIELD. WE THEN PROCEEDED VFR TO THE LOCATOR OM FOR THE APCH (ILS RWY 20). WHILE DSNDED AND FLYING TOWARD THE LOCATOR OM; WE COULD SEE A RWY LINE UP APPROX 200 DEGS. WHEN WE GOT TO 2000 FT I LEVELED THE ACFT AND BEGAN A TURN FROM A MODIFIED R BASE TO FINAL INSIDE THE LOCATOR OM AND CALLED FOR THE LOC TO BE TUNED AND IDENTED. WHILE ON FINAL THE CAPT WAS UNABLE TO TUNE AND IDENT; HOWEVER; WE HAD A GOOD VISUAL ON THE RWY. WE HAD NO REASON TO BELIEVE WE WERE ALIGNED WRONG AND THAT WE WERE GOING TO THIS UNCTLED AIRFIELD BECAUSE WE WERE HDG ABOUT 200 DEGS AND EVERYTHING LOOKED GOOD. IT WASN'T UNTIL SHORT-SHORT FINAL THAT WE SAW 18 ON THE RWY AND REALIZED WE WERE LNDG AT THE WRONG ARPT. THE ARPTS ARE ONLY ABOUT 10 MI APART WITH THEIR MAIN RWYS BOTH RUNNING IN THE SAME BASIC DIRECTION. FROM THE DIRECTION WE WERE APCHING FROM (W) THE FIRST RWY WE SAW APPEARED CORRECT. BOTH THE CAPT AND I WERE UNFAMILIAR WITH THE CLOSENESS OF THE 2 ARPTS. SUPPLEMENTAL INFO FROM ACN 241388: THE MARION WX WAS RPTED AS 10000 FT OVCST; 10 MI VISIBILITY WITH LIGHT RAIN. THE FO NOW HAD A GOOD VISUAL OF THE RWY OUT HIS R SIDE WINDOW. WE CONTINUED THE APCH AND I TUNED NAV #1 TO THE RWY 20 LCL AND AS BEFORE COULD NOT HEAR THE LOC IDENT. WE CONTINUED IN OUR APCH; I AGAIN LISTENED FOR THE LOC IDENT ON BOTH NAV RECEIVERS AND AT THIS POINT CONCLUDED THAT THE LOC GND STATION WAS OFF THE AIR. CAPT HAD NOT FLOWN TO THIS AREA DEST PREVIOUS TO THAT MORNING. FO RELATIVELY NEW TO COMPANY WITH 2 PREVIOUS NIGHT TRIPS TO THE MARION AREA. FACILITIES -- DSNDING INTO AN UNCTLED FACILITY (NO TWR AFTER 8 PM). NO ATIS INFO. WX -- DEPARTED COLUMBIA AT DUSK DSNDING IN TO NIGHT AT DEST WITH LIGHT RAIN AND TSTMS IN AREA. COMPLACENCY -- CAPT PROBABLY RELIED TOO HEAVILY ON THE FO'S ABILITY TO RECOGNIZE AND PROCEED TO CORRECT ARPT IN MARION AREA.