If you can't change the vent arrangement (I'm reading that you are stuck in defrost), then it is likely a problem in your blender doors.
However, some grands (not all) have rear heater cores, so it is possible your front core is bad but your rear isn't. If that is the case you can try disconnecting the hoses to the your front heater core and hooking up a hose for a good flush.
I would not bother trying that if you can't adjust the vent arrangement though. In that case, your going to need to take your dash off and troubleshoot the vent chamber, which is a pain btw.
I had same problem...my heater control valve was plugged. but I was told it could be anyone of following things or it could be blender doors. or low coolant level or bubbles in line. hopefully this helps you.
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