If you have hot heat from the rear heater but not the dash, then the coolant is obviously hot enough. That leaves a flow problem; either from a blockage in the core or in the lines to it. Or the flow control valve isn't fully opening. Since adjusting the valve is easy and free, I would check it first. Unhook the control cable and check if the valve will move further open.
If the valve isn't the culprit, it is possible to flush the core from the engine compartment. So that would be my next move.
I will try adjusting the control valve, it is worth a shot. I would say that my heat is basically luke warm out of the front and it will as another noted "cook a burrito" from the rear heater.
Zack