Update:

Managed to do some troubleshooting.

The floor flaps are actuated by one vacuum actuator. It's getting full vacuum, and moves fine. The trouble is, only the right door is connected...

I did not remove the plenum, but was able to move some of the ducting aside, and could feel the door. It feels like there is no connection, it's just free swinging. I think the whole dash has to be removed to get at the box, and I'm not about to do that.

I found that a bent coathanger could be pushed up through a hole in the plenum, holding the door open.

The coathanger is not noticeable and will be a semi-permanent part of the car until warm weather, then I'll let the close for A/C season.

It's not the best fix, but it will work, and my toes will stop freezing.

Best Regards,
Jim