No ac on drivers side. Temp door on heater box assembly broken. Repair or trade?

What is your question?

Dealer repair estimate $3100 part /heater box assembly. $1400 labor. Cause of ‘damage’ unknown. 45000 miles dealer regular maintenance, no accidents, only reasonable solution is used part or trade in. What is the cause and should I get a used part. When turning temp control down from heat to cool, clicking noise like a water bottle for a few seconds then noise stops. Went for second opinion. The doors on these assemblies seem to be designed poorly, not just a Toyota issue but common with other manufacturers. The door cannot be purchased separately from the housing. New part is a $3000 assembly. Used part can get by with just the housing/case. Repair with new part seems absurd. Repair with used part seems like a crap shoot if I don’t know why this happens in the first place and I’ll get almost no warranty.

What makes this problem better or worse?

Nothing

How long has your 2015 Toyota RAV4 had this problem?

serviced in oct. 2018. Tried ac in July and problem was there