A B747-400 CAPT TAXIED ONTO AN INCORRECT RWY AFTER BEING DISTRACTED BY A SHOULDER HARNESS ISSUE. HE LOST SA BEFORE RESUMING ACFT CTRL FROM THE FO.

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GIVEN TAXI INSTRUCTIONS TO TAXI OUT OF THE RAMP; Y4 R TURN TXWY D TO HOLD SHORT ON TXWY K AND RWY 15L FOR RWY 15R. WE COMPLIED WITH THAT CLRNC; THEN WAS GIVEN CLRNC TO CROSS RWY 15L TO TURN R ONTO TXWY C TO HOLD SHORT RWY 15R. TXWY C RUNS BTWN THE 2 PARALLEL RWYS 15L/R. WE CROSSED RWY 15L WITHOUT INCIDENT; MADE THE TURN ONTO TXWY C. ALONG TXWY C I REALIZED I HAD NOT FASTENED MY SHOULDER HARNESS AND THAT MY HARNESS WAS LOCKED AND COULD NOT WORK IT LOOSE. I INSTRUCTED MY FO TO CONTINUE TO TAXI THE ACFT WHILE I CORRECTED THE SEATBELT ISSUE. ONCE I HAD IT CORRECTED I ASSUMED CTL OF THE ACFT JUST BEFORE THE TURN TO RWY 15R ON TXWY C. JUST AS I RESUMED CTL OF THE TAXI; WE WERE CLRED INTO POS AND HOLD RWY 15R WHICH WAS A L TURN. I MISTAKENLY TURNED R; HOWEVER; TO RWY 15L; AFTER THE TURN THE CTLR QUERIED US. I STOPPED IMMEDIATELY AND WE ASKED FOR CLRNC TO TAXI ONTO RWY 15L THEN TAXI DOWN TO TXWY K AND BACK ONTO TXWY C FOR RWY 15R. THE CTLR ASKED US IF WE COULD DEPART RWY 15L INSTEAD. WE HAD ALREADY CHKED THE TKOF DATA FOR BOTH RWYS AND KNEW WE WERE LEGAL FOR THE DEP AND THE DEP CLRNC WAS A VECTOR HDG. WITH ONLY MINOR MODIFICATIONS TO THE FMC (DEP RWY SELECTED) REQUIRED WE ACCEPTED A RWY 15L DEP AND DEPARTED WITHOUT FURTHER INCIDENT. IN THE FUTURE I WILL SECURE MY SEATBELT AND SHOULDER HARNESS PRIOR TO TAXI; OR STOP THE ACFT TO FIX THE ISSUE; AND FINALLY I WILL VERBALIZE AND CONFIRM ALL TURNS WITH MY FO BEFORE EXECUTING THEM. I FEEL THAT THE SECOND XFER OF CTLS WHILE RECEIVING THE 'POS AND HOLD' CLRNC DISTR MY FO; NOT ALLOWING HIM TO CATCH MY TAXI ERROR. SO IF I HAD GIVEN HIM THE OPPORTUNITY TO CONFIRM THE TURN VERBALLY BEFORE EXECUTING IT; WE WOULD NOT HAVE HAD THIS INCIDENT. ALSO; FROM A DESIGN STANDPOINT; I HAVE NOTICED THIS LOCKING MECHANISM TO BE A PROB. IF THERE WERE SOME TYPE OF DETENT ADDED TO SEND THE SHOULDER HARNESS INTO LOCK; THIS WOULD BE IN THE INTEREST OF SAFETY.