Probably programmer, It needs to be checked out.
I had the same problem....the easiest way to know is to safely jack the vehicle up, run it until it's hot then follow the rubber coolant hose from rear heater to the engine. You will likely find it is cold all the way to the front. If you remove the fan shroud etc you will see a coolant distribution valve ( there are two) plastic and avialbale from GM. They have vacum connections on them. Check to be sure the connections are ok. If so the valve is likely stuck in the closed position prohibiting any coolant flow to the rear heater from the engine cooling system. The valve ( I would replace both while there since they are buggers to get at and change) . jk
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