If you can see but not necessarily reach the center vent actuator, either from the glove box or through the registers, maybe you can force it into open mode and hold it there with a wedge or wire. That way you can delay repair even 'til the fall
Can't think of why you'd need anything but the center vent for the next 4 months.
Sixto
91 300SE
87 300SDL
83 300SD