Blend doors are on units that have a separate temp control or that do defrost. Never seen a blend door on a rear ac unit from a conversion company. They are slaves to the front controls and are usually either all heat or all cool with their own fan control only. If it's a factory (GM) unit maybe it's different.
The coil needs only two things to remove heat. Air movement and refrigerant movement. If the fan is moving air well then you have a lack of refrigerant flow.
Sounds like a TXV problem.(Thermostatic Expansion Valve) The rear units have TXV controlled coils.
If it ever had a compressor fail, any lines replaced or a complete loss of charge, it could have a clogged TXV.
Is the rear unit mounted on the floor and ducted to the ceiling or is it mounted high? The floor mount unit lines can also become oil logged if the TXV is faulty/clogged.
Most mechanical repair shops do AC work as a sideline. Take it to a certified Automotive AC shop.
Last edited by CatFish; July 22nd 2022 9:34 am.