SF340 FLT CREW INTERCEPTS WRONG LOC AFTER CHANGE IN RWY ASSIGNMENT. ERROR IS QUICKLY NOTED AND CORRECTED BY THE FLT CREW AND ATC.

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WHILE BING VECTORED FOR APCH INTO IAH OUR ACFT WAS ASSIGNED THE ILS TO RWY 26R. WHEN VECTORED ONTO BASE LEG APCH CTLR ADVISED CHANGE OF RWY TO ILS RWY 26L. NEW LOC FREQUENCY WAS INPUTTED INTO NAV RADIO AND A BRIEFING WAS CONDUCTED AS TO THE CHANGE IN FREQUENCY AND PERTINENT CHANGES FOR THE APCH. SHORTLY AFTER FINISHING BRIEFING; THE ACFT AUTOPLT CAPTURED AND BEGAN TRACKING THE INBND COURSE. WHEN CROSS CHKING THE ACFT INSTRUMENTS THE FO AND I REALIZED THAT THERE WAS A DISCREPANCY BETWEEN OUR INSTRUMENTS. AS THE PF; MY INSTRUMENTS SHOWED PROPER INTERCEPT OF THE COURSE; WHILE THE FO'S INSTRUMENTS INDICATED A FULL-SCALE DEFLECTION FROM THE COURSE. IT WAS SOON REALIZED THAT WHEN I HAD INPUTTED THE NEW FREQUENCY IN MY NAV RADIO I HAD NEGLECTED TO SELECT IT TO ACTIVE AND THE ACFT AUTOPLT HAD INTERCEPTED THE ILS RWY 26R LOC COURSE INSTEAD. I IMMEDIATELY DISCONNECTED THE AUTOPLT AND TURNED THE ACFT TO JOIN THE ILS RWY 26L FINAL APCH COURSE. HOUSTON APCH AT THE SAME MOMENT HAD JUST NOTIFIED US THAT WE HAD INTERCEPTED THE WRONG LOC. THE FO ADVISED APCH THAT WE HAD TURNED TO INTERCEPT THE LOC FOR RWY 26L. FINAL APCH COURSE FOR RWY 26L WAS JOINED WITHOUT FURTHER INCIDENT. HIGH COCKPIT WORKLOAD CONTRIBUTED TO THIS EVENT. IN A SHORT AMOUT OF TIME NEW FREQUENCIES WERE ADDED AND A NEW BRIEFING NEEDED TO BE COMPLETED; CAUSING A BREAK IN THE ROUTINE AND FLOW OF DUTIES. IN ADDITION; FOR A BRIEF MOMENT; WE AS A CREW RELIED TOO MUCH ON THE AUTOPLT WHILE CONDUCTING THE NEW BRIEFING. DURING THIS TIME I WAS NOT MONITORING THE ACFT AND ITS INSTRUMENTS CLOSELY ENOUGH; CAUSING A LACK OF SITUATIONAL AWARENESS. IN THE FUTURE; I WILL HAVE THE NON-FLYING PLT BRIEF THE CHANGES AND MAKE THE APPROPRIATE INPUTS ON BOTH NAV RADIOS WHILE I CONTINUE TO MONITOR THE ACFT AND THE CHANGES BEING MADE.