What could be wrong with my heater?

I have a 2015 Nissan Rogue and the heater works on the passenger side, but blows cold are on the drivers side. Took it to a dealership, and they told me I need the heater box replaced. $2000. Call Nissan nothing they can do for me. Warranty expires 4,000 miles ago. They the dealership wanted to sell me a 2018 NIssan Rogue, who's to say it has the same kind of heater box in it.

I don't understand why Nissan doesn't have a recall on the heater/ac problems. especially with the 2015 Nissan Rogue.

Sounds like a door on thee underside/inside of the dash. When you turn that side on from defrost can you hear what would be a door opening under/inside the dash? Make it quite in the car and listen for the air mix door opening or closing. Here is exploded view of heating and cooling system. Anything with link, door or motor can be a possible problem. Like number 5-11 or 25-35 in picture 3. http://www.nirogue.com/heating_and_cooling_unit_assembly-1002.html