Funny you should mention it, just last weekend, mine seemed to close too after multiple years of being lightly rigged open (explained below).
To fix, it was just a matter of popping the glove compartment out after removing the two piece fasteners and carefully letting the glovebox light through in order to gain access.

I used my right hand reaching through to carefully remove the right half of the rubber vent tube to be able to reach in and flip down the vent.

I turned the engine on while I held the vent flap open and with a finger flick, nudged the center actuator arm so it sits above the open vent flap.
The end of the actuator arm has a wider end, and if it's over the open flap, it seems to just slide over the open vent flap.
I turned off and on the engine multiple times with my hand in there to make sure the actuator arm did indeed sail over without closing the flap.

Normally/from the factory, the open flap top center has a matching hole/circle where the actuator arm connects via pin. On my car, that pod is on the weak side, but not enough trouble to justify replacing it, as it's a bear to do from just about all accounts I've been able to find here.
If I need to adjust that flap, I just do it manually and it seems to hold in the desired position for at least a year. The fix I do takes about 10 min. max.
As my a/c works, and it's a window heavy/sunlight streaming TD, the center cooling is appreciated.

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85' 300D No inspection, No registration fees, Cheap insurance
"If my calculations are correct, when this baby hits 88 miles per hour, you're going to see some serious %$&^."