Heater vent flap

So the flap never worked when I got the car, and it came with AC, disconnected.

I removed the AC system when swapping the engine, just don't need it here in the winter.

But the flap in the heat system don't seem to function. I get a little blowing up through the defroster. The fan works great.

Any ideas where I should start looking?

i assume you are taking about the cable operated flap on the plenum. watch to see if the cable moves the lever in the plenum, if the lever moves and the heat doesnt come on, put in aniother plenum box.
if you are talking about the mode doors for defrost, vent & floor, those are vacuum operated motors, a quick test to se if the motors are working would be to remove the switch assembly and use vacumm hoses from the black hose to the others and see if the vacuum motors operate.
even quicker, disconnect the vacuum source for the climate control under the hood and put the fan on, the air should blow out from the defrost vents only good luck..

Thanks for that, yah I'm talking about the mode doors.
I think there are two vacuum motors, right?
I know about the one right behind the glove box. It doesn't work but I see it's hooked up to an interior flap, which I assume is the recirculation vent.

I think these diagrams on my web site might help you out.
AC/HEATER DOOR DIAGRAMS

Very cool, thanks!