So here'* the latest issue with the never ending learning experience that is my 99 Lesabre. One night I noticed the heater was blowing from the vents only even though I had it set to front defrost. There is no air blowing from the defroster duct whatsoever and instead continues to flow from dash vents. All other settings seem to work correctly. I replaced the electronic heat/AC control unit thinking there was some internal short but the issue persists. Also, with key on engine off the defroster duct does indeed blow air but as soon as I start the engine it transfers back to the vents. I've had to wipe the inside of my windshield with a towel for any visibility the past week so any help is appreciated.

Have you tried using the old controller again and see if that made any difference?
Have you tried hitting the off button first then hit defrost?
Normally I would say it may be a blend door actuator problem, but am confused as why with ign on without the car started, would you then only have defrost air...IDK.
I believe there is a way with a TechII scanner to do a diagnostic on system.
Hopefully others will chime soon as well.

Yes after playing with the new unit I reinstalled the old one. I also have tried hitting off then defrost.

Im thinking of pulling the control unit and metering the wires to find what'* doing what. If there'* a dedicated wire for defrost I can determine whether or not its even activating and run a relay to turn it on and bypass the whole thing. Not a professional fix but I have limited time and no warm garage to work in

I'm thinking this is all electrical rather than vacuum?

If it is, then you check the "mode" actuator to see if it moves when commanded...