Somewhere in the path of the air that the rear blower motor blows
Behind the glove box
Failed blower motor, resistor or switch.
The resister is right behind the glove box.
There are similar, but not the same.
Your truck has both. the resistor is between the switch & blower.thje resistor controls the blower speed
Perhaps resistor packs on blower motors is defective and need replacement
You lower the glove box lid by releasing the catches on the sides. You unplug the resistor and remove the two 8mm hex head screws.
With great difficulty!
The blower motor resistor is mounted in the heater box directly behind the dash glove box. Remove the contents of the glove box and push in on both sides of the glove box stops. The glove box door will fold down exposing the resistor. Remove the two screws holding the resistor in the heater box to remove the resistor.
You need to replace blower motor resistor.
The resistor is under the passenger dash just left of the blower motor
replace the blower motor resistor