Flush the heater core. Have found that the cores flow is different than you would think. It is possible for the coolant to go in and only flow in tank that has the two hoses never making it across the whole core. JMO
If the heater core flows the same as the one in my 03 Ram, that's the problem. Assuming the heater box is similar, it has a divider in it that isolates each side of the heater core. The way the core flows, one side can plug up while the other side still flows. My 03 did the same thing, cold on the driver's side, hot on the passenger side.
Kevin
