My 1984 GL heater blows cold and both hoses are hot. Only cold air comes through like not being directed through the core, as if outside air is the product. I pulled the covers; above feet and over the linkages and saw the link to the blender but that is as far as I could see. When turning heat selector, about 3/4 of the way, it gets suspiciously easy and produces no heat but just for a brief time, maybe 30 seconds and stops, only cold. All manuals show murky pictures if anything at all. It is like I have to wait until I see another wrecking yard subject I can dissect to learn from.

I can only speak for the 86 HVAC system, which is vacuum actuated... but I know which wires to pull. The blend door is on the drivers side right under the stereo console.

I have a Chilton manual for that year, you need me to take a look?

check that the bypass valves cable is moving properly.  It's above the gas pedal.

If it's not opening, the warm coolant is bypassing the heater core.