I have a 2010 GMC Savanah Van (3500) with a 6.0 liter motor. 1 of the heater hoses is hot going into heater core and the other is cold coming out. Engine is at operating temperature and I have no heat in the truck. Is there any other possibility other then a plugged heater core?? I will attempt a backflush but I am wondering if there is any other possibilities before I do the backflush?