ACFT EQUIP PROB. GROSS HEADING ERRORS ON BOTH CAPT AND FO'S HSI'S AND RMI'S. CAUSE UNKNOWN; BUT POSSIBLE INTERFERENCE FROM LCL RADIO TWRS.

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MAGNETIC INTERFERENCE CAUSING DRAMATIC HEADING ERRORS ON BOTH CAPT AND FO'S HSI'S AND RMI'S. LOCATION: APCH TO RWY 21 AT GEG; VISUAL APCH WITH ILS BACKUP FREQ 111.1. WE WERE ON A RADAR VECTOR OF 320 DEGS AT 6000 FT ASSIGNED AT APPROX 1/4 NM E OF NUMEROUS TWRS THAT ARE DEPICTED AS 4549 FT ON APCH CHART ILS RWY 21. THE INST COMPARATOR LIGHT WAS OUR FIRST INDICATION OF THE HEADING ERRORS. THE CAPT'S HSI AND THE FO'S RMI READ 30 DEGS TO THE R OF OUR ACTUAL HEADING. THE FO'S HSI AND THE CAPT'S RMI READ 30 DEGS TO THE L OF OUR ACTUAL HEADING -- NO OTHER FLAGS. AS WE INTERCEPTED OUR VISUAL GLIDEPATH (4000 FT) AND LINED UP STRAIGHT-IN TO RWY 21 (COURSE 208 DEGS) THE CAPT'S HSI READ 240 DEGS AND THE FO'S HSI READ 180 DEGS. AS WE APCHED TOUCHDOWN ALL HEADING INDICATORS WERE TURNING BACK TOWARD OUR REAL HEADING. AND; BY THE END OF OUR ROLLOUT ALL HEADING INDICATORS WERE BACK TO NORMAL. THIS WAS OUR SECOND LEG IN THE ACFT AND THIS WAS THE FIRST AND ONLY PROB THAT WE HAD ENCOUNTERED. AS THE WX WAS CLR FROM GEG TO SLC WE ELECTED TO TAKE THE ACFT BACK TO SLC AND WE DID NOT HAVE ANY PROBS -- THE NAV SYS WAS NORMAL. POSSIBLE CAUSE:?? INTERFERENCE FROM RADIO TWRS; PAX CELL PHONE OR OTHER EQUIP. THE FLT ATTENDANTS DID A QUICK RUN THROUGH THE CABIN ON TAXI IN AT GEG BUT COULDN'T FIND ANYTHING THAT MIGHT HAVE CAUSED THIS ANOMALY.