Remove your heater hoses & reverse flush out the heater core. I literally just did this yesterday! I had calcium deposits in my heater core from the previous owner using more water vs coolant. 230K on my D2 so no biggie. I bought 5ft of 5/8 heater hose, cut off the old heater hoses taking care not to tug on the heater core lines (you DO NOT want to damage the heater core lines/O-Rings). Flushed out the heater core backwards to push the crap out vs thru it, removed the 8mm bolt securing the second heater metal line to the other, carefully rotated it out of the way, cleaned off the metal line ends, slid the 5/8 line on it with a little WD40 to make it slid on easier, then I cut to fit the line. I did not put the heater core lines back into their OEM criss cross pattern. I went with the RRC setup straight in/out. Once done with the inner line rotate the outer lane back into position, and re-install the 8mm bolt, then slid the outer 5/8 hose on, and cut to fit as well. Secure them up with new hose clamps, and boom you’re done.When flushing the core I’d run it backwards at first and let it flow until it’s good and clear then flush the other way for good measure. After that run the D2 and check for any leaks, you should start getting heat, and the in/out hoses will be warm (I checked mine before I did any work and only one was warm. The other line was stone cold). Also for maximum heat make sure ECON mode is selected so the AC compressor doesn’t run. While my heater core was clogged I had zero cooling issues, just zero heat.

That sounds more like a blend door motor flap inside the HVAC unit itself not opening to allow warm air into the cabin. I verified my issue by feeling the heater hoses themselves. They’ll both be warm if it’s not clogged. If one is hot & one is cold it’s clogged.