FLT CREW OF MD83 ENCOUNTER HDG ANOMALIES BTWN THE NAV DISPLAYS AND THE REPEATER RMI'S. ULTIMATELY DISCOVER THE TRUE-NAV SWITCH HAD BEEN ACTIVATED; THUS DISPLAYING TRUE HDG ON THE NAV DISPLAYS.

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APCH CTL ASSIGNED A HDG DOWNWIND FOR SEQUENCE TO THE FINAL APCH. WE COMPLIED AND THEY ASKED WHAT OUR HDG WAS. THERE WAS AN APPROX 15-20 DIFFERENCE WITH THE NAV DISPLAY AND THE RMI. BOTH THE CAPT AND THE FO'S RMI'S AGREED. BOTH NAV DISPLAYS AGREED; BUT THE NAV DISPLAYS WERE THE PROB -- THEY WERE INACCURATE. CHKED WET COMPASS. DID NOT SEE A 'HDG' ON THE FMA; THAT WOULD HAVE DENOTED A CONFLICT WITH THE 2 SYS. WENT TO THE BOOK TO SEE IF THIS ISSUE WAS ADDRESSED AND IT WAS NOT. BEGAN AN AUTO-COUPLED ILS RWY 10R AND THE LOC CAPTURE DID NOT PROPERLY ALIGN AND WE HAD A FULL SCALE NEEDLE DEFLECTION. REQUESTED A TURN OUT OF THE FINAL APCH COURSE; CLBED TO 3000 FT. ATC WAS AWARE OF OUR COMPASS CONFLICT. COMMENCED OUR SECOND APCH USING RAW DATA TO A SUCCESSFUL LNDG. WHILE CONSTRUCTING MY LOGBOOK ENTRY AND FURTHER DISCUSSING THE SIT WITH THE FO; I DISCOVERED THAT THE GREEN; TRUE/MAGNETIC LIGHT WAS ILLUMINATED TO TRUE; THUS ELIMINATING VARIATION ON OUR NAV DISPLAYS. I PUSHED THE BUTTON TO MAGNETIC AND THE SYS RETURNED TO NORMAL. IT HAD BEEN TURNED ON ACCIDENTALLY. THIS IS AN ITEM THAT SHOULD BE ADDRESSED. THIS COULD BE A VERY SERIOUS ISSUE IF THE VISIBILITY WAS AT MINIMUMS.