ACR CREW NEGLECTS TO CHANGE FREQ AND LANDS AT SEATTLE; WA; WITHOUT CLRNC.

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CLRED FOR A VISUAL APCH TO RWY 34L AT SEA; WE WERE NOT SWITCHED TO TWR BY THE APCH CTLR OR IF WE WERE; WE MISSED THE CALL. WE WERE FOLLOWING AN ACFT IN FRONT OF US AND ESTABLISHING THE PROPER SPACING PREVENTED THE COPLT OR MYSELF FROM REALIZING WE HAD NOT BEEN SWITCHED TO TWR OR THAT WE HAD MISSED IT. WHILE WE HAD SUFFICIENT CLRNC BEHIND THE ACFT IN FRONT OF US; OUR FIRST REALIZATION THAT WE HAD NOT SWITCHED TO TWR; CAME WHEN WE CALLED TWR HOLDING SHORT OF RWY 34R AND WERE TOLD WE WERE STILL ON APCH. I'VE DEVELOPED THE HABIT OF NOT TURNING ON THE NOSE GEAR LNDG LIGHT UNTIL I'VE GOTTEN A LNDG CLRNC AND SURE ENOUGH WHEN I LOOKED UP THE NOSE GEAR LIGHT WAS OFF. SUPPLEMENTAL INFO FROM ACN 469382: THE CAPT DISCONNECTED THE AUTOPLT. ATC VECTORED THE ACFT TO A 250 DEG HDG AND QUERIED 'DO YOU HAVE THE ARPT IN SIGHT?' THE CAPT ACKNOWLEDGED AND SAID 'YES WE GOT IT.' I REPLIED ATC'S QUERY WITH AN AFFIRMATIVE. ATC THEN SAID 'FLT XXX YOU'RE CLRED FOR THE VISUAL APCH RWY 34L; CROSS 5 MI FROM SEA AT OR ABOVE 2000 FT; CONTACT TWR 119.9.' I READ BACK THE CLRNC AND DIALED THE FREQ ON THE #2 RADIO. THE CAPT CALLED FOR FLAPS; AND I DIVERTED MY ATTN TO HIS FLYING AND COMMANDS. HE CALLED FOR GEAR AND LNDG CHKLIST. I WAS CONCERNED WITH PROPER PROCS AND THE PREVIOUS RESTR ISSUED BY ATC. UPON LNDG; WE TAXIED OFF THE RWY AND CALLED 'TWR; FLT XXX IS CLR OF THE ACTIVE.' SEA APCH ANSWERED BY SAYING; 'FLT XXX YOU ARE ON APCH FREQ.' I BELIEVE THIS INCIDENT WAS CAUSED BY TASK SATURATION AND A LITTLE FATIGUE SINCE WE HAD AN EARLY MORNING DEP OUT OF DCA. TASK SATURATION DUE TO FLYING THE AIRPLANE MANUALLY; WHICH REQUIRED THE PNF TO MONITOR THE FLT AND SET ALL ALTS AND HDGS GIVEN BY ATC. OUR FLT MANUAL RECOMMENDS THE USE OF THE AUTOPLT TO THE MAX EXTENT POSSIBLE. THE CAPT'S ELECTION TO DISCONNECT THE AUTOPLT EARLY DURING THE APCH; COMBINED WITH FATIGUE AND PNF SATURATION DUE TO MULTIPLE ATC INSTRUCTIONS AS WELL AS CABIN DUTIES WERE CONTRIBUTING FACTORS. THIS INCIDENT COULD HAVE BEEN PREVENTED BY USING THE AUTOPLT A BIT LONGER AND BY APCH CTL/TWR INQUIRING ABOUT OUR FREQ CHANGE; IE; 'FLT XXX ARE YOU WITH ME?'