Air blend door

Does the vehicle have dual zone climate control, where the driver side and passenger side have different control? Sounds like a blend door issue on the driver side. Why did you replace the passenger side, was it blowing hot too when AC was on?

Usually when one side is hot and the other is cold, and each side is controlled separately, the problem is a blend door not working.
The blend doors are usually controlled by small electric motors.
You might research "blend door" online and see if anything pops up.

It is likely that one or more of the blend doors or their actuators are stuck. Before any repair action is done, try allowing the HVAC system to recalibrate itself. To do this, turn all HVAC controls and switches to their OFF or CCW postions, then disconnect the battery for about 10 minutes. Reconnect the battery, but do not touch any of the HVAC controls or switches for about 2 -3 minutes. If this does not cure the problem, the HVAC system will need to be diagnosed. Access to the blend doors and actuators may require some disassembly of the center dash area.

Replace the door blend actuator behind glove box, approx $150 for part, Everyone I know with dual control on the silverado's have this problem.

To correct this problem u will need to replace the air temperature control blend door and the electric blend door actuator inside the dash, contact your local dealer for parts, they are dealer only parts

sounds like you have dual zone climate control in your truck. Most likely the actuator that controls the temp door in the HVAC box

this problem is caused in almost all cases by a failed actuator that moves the door insiode the heating/ac duct to change the outlet temperature, it is called the Air Temperature Control Blend Door actuator.

U have a stuck air temp control blend door, either the actuator motor is defective or the door is warped and has jammed, one thing is that u must remove the dash to rcheck replace these parts.

ase mechanic:
there are separate actuators for the blend doors. one for heat and one for cool. they work opposite and in conjuction with each other, if that makes any sense. ones open, ones close, for each different climate. sounds like the the cold one isn't opening in the rear.

the blend doors have broken...you will need to remove the heater/ac box to replace them