Coolant Flush Questions

there is the possibility that "water bypass pipe No. 1" is plugged where it meets the "V" under the lower intake if she is a 3.4. If the coolant flow can't get back into the system the heater core won't work.
A quick test is to splice in clear tubing... bypass the heater core... and watch what happens as she warms up. Put the radiator cap on the first notch, pressure will not build up, this "should" keep coolant from over flowing, hitting fan and spraying all over the place, the point is the plastic tubing won't hold pressure.

 

You can definitely reach its plastic box with a fully assembled dash. It's the black box to the right of the accelerator peddle.

Also, from passenger side you can see where the console control cables terminate under the heater/blower boxes.