Help! '93 525iT IHKR heater - air distribution

Ok, as the header says, I'm working on a new to me '93 525iT. PO reported that he had had the heater core replaced and heat hadn't worked properly since. I can feel heat and hear the fan doing it's thing, but air just sort of leaks out indiscriminately. Started pulling parts and found the outside air bowden cable unclipped from the slide controls - broken female retainer on the slide control. Not the problem. Got to the point that I pulled the microfilter, left it's cover off, and started the fan - bunches of air gets pushed backwards out the microfilter cover opening. For some reason neither the heat nor defrost flaps are opening, though the bowden cables move smoothly and the levers on the heater box move back and forth. I pulled the stepper motor onthe right side of the heater box and it makes little short spins as I play with the heat/defrost sliders. I suspect, though I don't know, that the stepper motor mixes air to reach a given temperature. This system is completely new to me - I've been to the German site that Shogun recommends, but it doesn't seem right on point - different system. Can anyone hazard a guess as to what the problem might be here? Hate to pull all the clips on the heater box if it can be avoided.
Thanks for any help or further troubleshooting suggestions.
Tom Walrod