Lucky, the little door is a damper door to allow cold air from freezer in to fresh food section. It opens and closes, this is determined by the tempature sensor. When the sensor feels to warm it’ll send voltage to the damper to open. Then to close. You could have a failed sensor or a failed damper. The problem is the back panel. I believe the back panel is nothing more that an air vent with the damper desperate from the panel. Or is the damper part of the back panel? I couldn’t tell in the parts breakdown. The back panel is no longer available. I found no other damper on this model. The sensor is part #WR55X10025. It’s under $25 and worth a try. Cross your fingers and ope all you had wrong is the sensor. Good luck.