2011 4R heater only blows hot from driver's side

Had the cooling system flushed and specifically the heater core flushed as I had no heat. The shop said gunk oozed out and then water flowed through the core. The mechanic drove it home and he said it initially had hot air that gradually diminished but only from the driver's side. They suspect it's a blend door or blend door control problem. Has anyone seen this or had experience with this in the past?

 

"gunk oozed out" doesn't sound very promising.

I drove an '85 Supra without heat for 2 winters before I realized that the motor that controls the air temperature was just shot. I figured out how to manually bypass it, and had heat again.