1996 Toyota 4Runner NO HEAT FRONT VENTS BUT OK AT BACK SEAT

HEADGASKET-RADIATOR-HOSES -THERMOSTAT ALL REPLACED LAST YEAR. MAJOR RUST BUILD UP PRIOR TO HEADGASKET GOING UP. THE MECHANIC HAS BACKFLUSHED 8 TIMES ALREADY AND IT WORKS OK A FEW DAYS THEN STARTS AGAIN. NO HEAT ON ANY SETTING FRONT. BUT CAN FEEL HEAT IN BACK SEAT AREA DUCT EXIT. I WANT TO CHANGE HEATER CORE BUT MECHANIC SAID IT CANT BE THAT. BUT SO FAR CANNOT STOP THE PROBVLEM