Heater duct door switching problem

[bradleyheathhays]

Have an '02 Taurus w/ 103k miles and about 6 months ago all the air, heat or a/c, started blowing only out of the defrost vents at the bottom of the windshield. All the settings work correctly except air has stopped blowing from the floor and dash vents entirely. I know the fix on this is probably easy, just getting to it is gonna be hard. I've had the dash off my '11 Crown Vic to replace the leaking heater core so I've got a little experience doing this sort of thing.

Can any describe to me what switching door in the vent system has gone bad and how I might approach fixing it?

Thanks