A B737-300 REJECTED TKOF DUE TO TKOF WARNING HORN SOUNDED. FOUND FLAP INDICATOR CIRCUIT BREAKER TRIPPED. BREAKER TRIPPED BY FO'S SHOULDER HARNESS.

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TWR CLRED THE FLT INTO POS ON RWY 11L. CAPT CALLED 'BELOW THE LINE.' HE PUSHED UP THE THRUST LEVER TO CHK TKOF WARNING HORN. WHEN WE REACHED STEP #2; THE COCKPIT DOOR WAS NOT LOCKED. HE ASKED ME TO MOVE THE SWITCH ON THE DOOR TO THE LOCKED POS. I REMOVED MY SHOULDER HARNESS TO REACH BACK AND SECURE THE LOCK. WE CONTINUED THE 'BELOW THE LINE' CHKLIST. MOMENTS AFTER WE WERE CLRED FOR TKOF AND UPON ADVANCING THE THRUST LEVERS WE GOT THE TKOF WARNING HORN. THE REJECTED TKOF WAS ACCOMPLISHED WITH ALL APPROPRIATE PROCS. UPON EXITING THE RWY; WE DISCOVERED THAT THE 'LEADING EDGE FLAP INDICATOR' CIRCUIT BREAKER WAS TRIPPED. NOTING ITS LOCATION; AND ME NOT GUARDING MY SHOULDER HARNESS WHEN I RELEASED IT TO LOCK THE DOOR; I AM CERTAIN THAT MY SHOULDER HARNESS TRIPPED THE CIRCUIT BREAKER. WHAT I SHOULD HAVE DONE: 1) CHKED THE COCKPIT DOOR LOCK BEFORE PUSHBACK FROM THE GATE. 2) KNOWING THAT THE SHOULDER HARNESSES CAN TRIP CIRCUIT BREAKERS; I SHOULD HAVE GUARDED THE BELTS BETTER WHEN I RELEASED THEM. 3) MOST OF ALL; WE SHOULD HAVE STARTED THE 'BELOW THE LINE' CHKLIST FROM THE BEGINNING ONCE WE CORRECTED THE UNLOCKED DOOR. THIS WOULD HAVE PROMPTED US TO ADVANCE THE THRUST LEVER AND CATCH THE PROB.