That old guy is complicated enough that we built our own analyzer to diagnose it. Our tester hooks in series with one of the two electrical connectors at the servo in the engine compartment.
All the functions of our tester can be done without it but some trial and error would have to be used as there are no voltage values in the manual.
Our tester monitors the feedback pot voltage (for relative servo position), it measures the polarity to the drive motor (internal; this shows which direction heat or cold that the servo is being driven, and probably the most important it has an over ride switch which controls the motor so that the servo can be manually driven to a different position.
Without this I would move the temp wheel and listen to the servo with a stethescope (or screw driver to ones ear). The servo should whir everytime a change is made. From your description you are either stuck in full heat or defrost. I would imagine that you have a bad amplifier (behind glove box). I would have condemned the servo first but you did say it was replaced?? Find that fuse on your wiring diagram (I don't remember where it is in the 123 car).