Hello and welcome to just answer. Let me see if I can help you with this. A radiator issue cannot cause this problem that you are describing. Something is wrong inside of the HVAC housing (heater box). When you change the mode dial, does the air switch to the positions you select? does the temp knob affect the air's temperature when you change it?
well to be honest with you, you do not need to remove the heater box and open it up to replace the blower motor on this model truck, so there should not have been any intervention with the blend door. Did the mechanic actually pull the box out and open it up?
Hmmm is right. It is possible that he damaged the blend door actuator i suppose. Another thing it could be is that there is air in the cooling system, and only half of the heater core is getting hot. This could of happened when the radiator was replaced, and when he refilled the cooling system, the air did not get purged correctly. Make sure the radiator is full.
I suppose he could have messed it up doing the blower replacement. Maybe you should have another tech double check his work. And even though he said he checked the coolant level, it needs to be rechecked just to be sure.