ac cool, freon, fan, belt ok. Is there a heater vent that could be stuck open? on 2003 Mitsubishi Montero Sport

My mechanic checked the freon, replaced a fan motor and belt. He then said that possibly there could be a heater vent that might not be closing all the way, which is allowing hot engine air to mix with the cold air coming from the vents. What is this and how can it be fixed?