Can be nasty.
Ther center duct and the defroster duct are on the same vacuum one closes with vacuum (defrost) and one opens. I think the same vacuum might work a portion of recirc also. A leak in one can put all of them in the off position. Off is max recirc, defrost, closed center vent.
Just so happens that I took some pictures last month of one that took more than a vacuum leak. The center duct vac element is inside the heater/AC box and the attacment had broken and been really rigged from the outside.
I am going to paste three pictures in three different posts (as I don't know how to do it in one). The first one in this post will be an overview with the dash removed and the upper housing removed. The second will be the housing on the dash showing the cracked housing at cntr duct element. The third will be back together with a piece of plumbers strap pop-riveted and epoxied to reinforce.
Bosch Master, ASE Master, L1
33 years MB technician