Date registered: Aug 2002
Vehicle: 96 G300DT
Location: Los Altos, CA (USA)
Mentioned: 10 Post(s)
Quoted: 112 Post(s)
It is certainly possible that it is the core. But first verify that the valve is opening. That white lever on the side of the valve is what open/closes the flow. See if it moves with the lever. Failing that take the valve off of the car and look through it.
You can check the core for flow by taking off the hoses and blowing through it. I'd work pretty hard at non-destructively clearing it out. Because ...
Changing the heater core is a big job. Most of the dash has to come out. Then the heater box has to be split to get to the core, fan, and the fan resistor. If you are in that far, replace all of them.