I had this problem a long time ago with my '05. As mentioned above, flushing the heater core alone, after a full system flush and reversing the heater core flow repeatedly will get that cleared.
In 2005 there was a brown sealant added to the coolant at the factory that caused quite a few problems. After I went through the flushing cycle a few times, I stopped replacing the suggested sealant and the heater core clogs ended.
A couple of years later, my son blew the head gaskets on the car, "mudding" and over revving the engine. After the heads were resurfaced and replaced, I noted a slight coolant leak outside of the back head near the #1 cylinder and rather than use the brown pellets suggested, I used alumaseal with no problems.