Heater Door Replacement '92 Altima

Car's heat stopped working about a week ago. Blows nothing but cold air.
Dealer says it's the "heater door" and it's a $500 part. Dash needs to be
taken out to replace and wants to charge almost $2000. Anyone had a similar
problem and if so, what was the diagnoses? Has anyone replaced a heater
door and if so, do you know the part number? Thanks - Russ