B737-300 CREW DID NOT INSURE AN MEL ITEM WAS ADDED TO THE DISPATCH RELEASE.

Narrative

THE CAPT'S FLUORESCENT BACKGND LIGHT WAS INTERMITTENT AND I CALLED MAINT IN RANGE TO HAVE IT REPLACED. WE HAD 40+ MINS ON THE GND (PLENTY OF TIME TO REPLACE UNIT). THE MECH SAID HE WOULD BE HAPPY TO MEL THE LIGHT; WHICH HE DID. IT WAS CLOSE TO PUSH TIME AND I FORGOT TO CALL DISPATCH TO AMEND THE RELEASE. I CALLED DISPATCH ON ARR IN ONT AND INFORMED HIM WHAT I HAD DONE AND HAD THE OUTBOUND RELEASE AMENDED FOR THE NEXT CREW; BUT IT WAS TOO LATE FOR MY FLT. I HAD ALREADY MADE THE BOO-BOO BY FORGETTING TO CALL IN ZZZ. THE DISPATCHER HAD NOT BEEN INFORMED BY MAINT EVEN BY THE TIME WE COMPLETED OUR FLT; SO THE NEXT RELEASE DIDN'T SHOW THE MEL ITEM. FACTORS: 1) I WAS SOMEWHAT FRUSTRATED WITH THE MECH'S LACK OF INTEREST IN REPAIRING THE LIGHT. 2) THE BACKGND LIGHT IS NOT AN EXTREMELY SIGNIFICANT ITEM. 3) I HAD ALREADY SIGNED THE AGENT'S COPY OF THE RELEASE; CHKED AND PUT MY COPY ON THE CLIPBOARD AND ACCOMPLISHED MY FLOWS FOR THE NEXT LEG. 4) NO MORE EXCUSES; I JUST DIDN'T THINK TO CALL THE DISPATCHER UNTIL WE WERE AIRBORNE; THEN DECIDED TO CALL WHEN WE LANDED.