I have looked and looked... I have studied the FSM and batee.com.

C68 controller or programmer issue, I think. When in econ or heat, if desired temp is above 62, the defrost door closes a little bit. Any more degrees warmer, and it closes all the way. This directs all the air to the windshield. I can’t get air blowing through the rear ducts (floor) in any mode. All the doors’ actuators are tested and mechanically sound. I have not seen a single troubleshooting procedure mention anything about the defrost door being affected in any way by the desired temp setting. The electric blend door operates correctly in all modes. The inner connector of the controller is weak, and sometimes I have to pluck the face of it to make the back lights come on with the headlights. It doesn’t always remember what speed the blower is manually set to, when the car is turned off and on.

Also weird: I can’t find a single mention of this specific model, that describes which solenoid is supposed to be hooked to which vacuum lines, so any help there would also be greatly appreciated. There was one person (Cliff Harris) that did a lot of his own electronic analysis, and mapped his six solenoids back in 2011, but the ‘89 has only five (apparently the white line was abolished by then, according to SunCr). At this point, that is the only reference I have. So I’m assuming L1: orange, L2: pink, L3: tan, L4: n/a, L5: red, L6: blue.

 

Not sure how I missed this yesterday, but I just stumbled on this page from batee.com: http://www.batee.com/corvette/acrepair/c68pgmrboard/. It's mostly way over my head on the electricals, but it does seem to confirm the solenoid-color map from my original post. I hope Bryan doesn't mind if I copy his images here. I only do it because sometimes when I'm reading a 20-year-old forum post, it is so annoying when someone's links are broken.

 

Pull the controller out and inspect the 2 140Ω resistors that I circled. Those resistors get hot, and result in the solder joints making poor contact on the back of the board. Re-solder them and reinstall and there is a 99.99% chance that your oddball door problems will vanish. I know this from experience...been there - seen that - done that.

The controller is located under the hush panel, directly above the gas pedal, mounted to the firewall,

 

Hi Red, I really appreciate the response. I hadn’t mentioned that I replaced those resistors, along with the two diodes and transistors related to the blend door circuitry. Didn’t make any difference. The electric blend door is doing what it’s supposed to do. I realize my original question might be a little rambly and hard to decipher, so I’ve got this restatement of it:

Is there any way that any of the vacuum solenoids would be actuated by any changes to the desired temperature setting?

I still have some parts from the batee.com C68 rebuild kit for the head unit, and I guess I’ll go ahead and replace those, but my intuition is telling me it won’t make a difference. All the lights and buttons, and the temp display work.