I wonder if you wouldn't be able to flush it by disconnecting the two lines on the engine side of the fire wall? Don't rely completely on this, but the two hard lines behind, (or below) the EGR valve are what I think, go to the heater core. They transition from pipe, to rubber hose between the EGR and the cruise control box. One is an inlet and one is outlet. Again, I'm not positive anything could be done from this side, but may be a good chance! Or maybe use suction on it? Might save a lot of headache tearing the whole dash out!